Distortion compensating apparatus

ABSTRACT

A distortion compensating apparatus for correcting the size of a distortion compensation coefficient in such a manner that a transmit signal that has undergone distortion compensation will not exceed the dynamic range of a DA converter. Specifically, before a distortion compensation coefficient h n+1 (p) that has been calculated by a calculation unit is stored in a coefficient memory, an assumption is made that distortion compensation will be performed using the distortion compensation coefficient h n+1 (p). Then it is determined beforehand whether a signal x(t)*h n+1 (p) that will be obtained by this distortion compensation will exceed the limit of a DA converter. If the limit will be exceeded, the size of the distortion compensation coefficient is reduced by a correction unit, the corrected distortion compensation coefficient is stored in the memory and the transmit signal is corrected using the stored distortion compensation coefficient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/737,196 filed on Dec. 14, 2000 now U.S. Pat. No. 7,012,969, andclaims priority under 35 USC 119 from Japanese Patent Applications2000-097092 filed Mar. 31, 2000 and 11-372885 filed Dec. 28, 1999, thecontents of which are herein wholly incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a distortion compensating apparatus and, moreparticularly, to (1) a distortion compensating apparatus capable oflimiting amplitude when it appears that control which exceeds outputlimits will be carried out and capable also of exercising a phasetracking operation even when amplitude has been limited, and (2) adistortion compensating apparatus so adapted that the amplitude of asignal fed back from a transmission power amplifier is controlled sothat a limit value will not be exceeded, and so adapted that distortioncan be compensated for in stable fashion.

Frequency resources have become tight in recent years and in wirelesscommunications there is growing use of high-efficiency transmissionusing digital techniques. In instances where multilevel amplitudemodulation is applied to wireless communications, a vital technique isone which can suppress non-linear distortion by linearizing theamplitude characteristic of the power amplifier on the transmitting sideand reduce the leakage of power between adjacent channels. Alsoessential is a technique which compensates for the occurrence ofdistortion that arises when a attempt is made to improve powerefficiency by using an amplifier that exhibits poor linearity.

FIG. 45 is a block diagram illustrating an example of a transmittingapparatus in a radio according to the prior art. Here a transmit-signalgenerator 1 transmits a serial digital data sequence and aserial/parallel (S/P) converter 2 divides the digital data sequencealternately one bit at a time to convert the data to two sequences,namely an in-phase component signal (also referred to as an “I signal”)and a quadrature component signal (also referred to as a “Q signal”). ADA converter 3 converts the I and Q signals to respective analogbaseband signals and inputs these to a quadrature modulator 4. Thelatter multiplies the input I and Q signals (the transmit basebandsignals) by a reference carrier wave and a signal that has beenphase-shifted relative to the reference carrier by 90° and sums theresults of multiplication to thereby perform quadrature modulation andoutput the modulated signal. A frequency converter 5 mixes thequadrature-modulated signal and a local oscillation signal to therebyeffect a frequency conversion, and a transmission power amplifier 6power-amplifies the carrier output from the frequency converter 5. Theamplified signal is released into the atmosphere from an antenna 7.

In mobile communications based upon W-CDMA and PDC (Personal DigitalCellular) techniques, etc., the transmission power of the transmittingapparatus is a high 10 mW to several watts, and the input/outputcharacteristic [distortion function f(p)] of the transmission poweramplifier is non-linear, as indicated by the dotted line in FIG. 46A.Non-linear distortion arises as a result of non-linear characteristics,and the frequency spectrum in the vicinity of a transmission frequencyf₀ develops side lobes, as shown in FIG. 46B, leakage into the adjacentchannel occurs and this causes interference between adjacent channels.More specifically, owing to non-linear distortion, there is an excessiveincrease in power that causes transmitted waves to leak into theadjacent frequency channel, as shown in FIG. 46B. The leakage power isdescribed on the basis of ACPR (Adjacent Channel Power Ratio). ACPR isthe ratio between the power of the channel of interest, which is thearea of the spectrum between the one-dot chain lines A and A′ in FIG.46B, and the adjacent leakage power, which is the area of the spectrumbetween the two-dot chain lines B and B′, that flows into the adjacentchannel. Such leakage power constitutes noise in other channels anddegrades the quality of communication of these channels. Such leakagemust be limited to the utmost degree.

Leakage power is small in the linear region (see FIG. 46A) of the poweramplifier and large in the non-linear region. Accordingly, it isnecessary to broaden the linear region in order to obtain a transmissionpower amplifier having a high output. However, this necessitates anamplifier having a performance higher than that actually needed andtherefore is inconvenient in terms of cost and apparatus size.

With an ordinary amplifier, power added efficiency in the linear regionis low, as indicated by FIG. 47. Power load efficiency, which is thepercentage (%) of the difference (Pout−Pin) between output power Poutand input power Pin with respect to the rated power of the amplifier, isthe portion given off as heat. To obtain the necessary transmissionpower, therefore, it is required that a large amount of power beconsumed. This is inconvenient in terms of power efficiency. Thus it isessential to use the amplifier in the non-linear region in order to holddown the amount of power consumed. As mentioned above, however,distortion increases and degrades the ACPR. A device that compensatesfor distortion of transmission power and enables use of an amplifier ina region of excellent power added efficiency is a wireless apparatus(linearizer) having a distortion compensating function. The Cartesianloop method and polar loop method, etc., have been proposed astechniques for effecting distortion compensation by feedback and thesemethods succeed in suppressing the distortion of power amplifiers.

FIG. 48 is a block diagram of a transmitting apparatus having a digitalnon-linear distortion compensating function that employs a DSP. Heredigital data (a transmit signal) sent from the transmit-signal generator1 is converted to I and Q signals by the S/P converter 2. These signalsenter a distortion compensator 8 constituted by a DSP. As illustrated inFIG. 49, the distortion compensator 8 functionally comprises adistortion compensation coefficient memory 8 a for storing distortioncompensation coefficients h(pi) (i=0˜1023) conforming to power levels0˜1023 of a transmit signal x(t); a predistortion unit 8 b forsubjecting the transmit signal to distortion compensation processing(predistortion) using a distortion compensation coefficient h(pi) thatis in conformity with the level of the transmit signal; and a distortioncompensation coefficient calculation unit 8 c for comparing the transmitsignal x(t) with a demodulated signal (feedback signal) y(t), which hasbeen obtained by demodulation in a quadrature detector described later,and for calculating and updating the distortion compensation coefficienth(pi) in such a manner that the difference between the compared signalswill approach zero.

The distortion compensator 8 subjects the transmit signal x(t) topredistortion processing using the distortion compensation coefficienth(pi) that conforms to the power level of the transmit signal x(t), andinputs the processed signal to the DA converter 3. The latter convertsthe input I and Q signals to analog baseband signals and applies thebaseband signals to the quadrature modulator 4. The latter multipliesthe input I and Q signals by a reference carrier wave and a signal thathas been phase-shifted relative to the reference carrier by 90° and sumsthe results of multiplication to thereby perform quadrature modulationand output the modulated signal. The frequency converter 5 mixes thequadrature-modulated signal and a local oscillation signal to therebyeffect a frequency conversion, and a transmission power amplifier 6power-amplifies the carrier output from the frequency converter 5. Theamplified signal is released into the atmosphere from an antenna 7.

Part of the transmit signal is input to a frequency converter 10 via adirectional coupler 9 so as to undergo a frequency conversion and thenbe input to a quadrature detector 11. The latter multiplies the inputsignal by a reference carrier wave and a signal that has beenphase-shifted relative to the reference carrier by 90°, reproduces theI, Q signals of the baseband on the transmitting side and applies thesesignals to an AD converter 12. The latter converts the applied I and Qsignals to digital data and inputs the digital data to a distortioncompensator 8. By way of adaptive signal processing using the LMS (LeastMean Square) algorithm, the distortion compensator 8 compares thetransmit signal before the distortion compensation thereof with thefeedback signal modulated by the quadrature detector 11 and proceeds tocalculate and update the distortion compensation coefficient h(pi) insuch a manner that the difference between the compared signals willbecome zero. The transmit signal to be transmitted next is thensubjected to predistortion processing using the updated distortioncompensation coefficient and the processed signal is output. Byrepeating this operation, non-linear distortion of the transmissionpower amplifier 6 is suppressed to reduce the leakage of power betweenadjacent channels.

FIG. 50 is a diagram useful in describing distortion compensationprocessing by an adaptive LMS. A multiplier (which corresponds to thepredistortion unit 8 b in FIG. 49) 15 a multiplies the transmit signal(the quadrature-modulated signal) x(t) by a distortion compensationcoefficient h_(n−1)(p) and applies its output to a transmission poweramplifier 15 b having a distortion function f(p). A feedback loop 15 cfeeds back the output signal y(t) from the transmission power amplifier15 b and an arithmetic unit (amplitude-to-power converter) 15 dcalculates the power p[=x(t)²] of the transmit signal x(t). A distortioncompensation coefficient memory (which corresponds to the distortioncompensation coefficient memory 8 a of FIG. 49) 15 e stores thedistortion compensation coefficients that conform to the power levels ofthe transmit signal x(t). The memory 15 e outputs the distortioncompensation coefficient h_(n−1)(p) conforming to the power p of thetransmit signal x(t) and updates the distortion compensation coefficienth_(n−1)(p) by a distortion compensation coefficient h_(n)(p) found bythe LMS algorithm.

A complex-conjugate signal output unit 15 f has the output of thefeedback system 15 c applied thereto. A subtractor 15 g outputs thedifference e(t) between the transmit signal x(t) and the feedback(demodulated) signal y(t), a multiplier 15 h performs multiplicationbetween e(t) and u*(t), a multiplier 15 i performs multiplicationbetween h_(n−1)(p) and y*(t), a multiplier 15 j multiplies the output ofthe multiplier 15 h by a step-size parameter μ, and an adder 15 k addsh_(n−1)(p) and μe(t)u*(t). Delay units 15 m, 15 n, 15 p add a delay timeto the input signal. The delay time is equivalent to the length of timefrom the moment the transmit signal x(t) enters to the moment thefeedback (demodulated) signal y(t) is input to the subtractor 15 g. Thecomplex-conjugate signal output unit 15 f and the multipliers 15 h, 15 iand 15 j construct a rotation calculation unit 16. A signal that hassustained distortion is indicated at u(t). The arithmetic operationsperformed by the arrangement set forth above are as follows:h _(n)(p)=h _(n−1)(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=h _(n−1)(p)x(t)f(p)u(t)=x(t)f(p)=h* _(n−1)(p)y(t)P=|x(t)|²where x, y, f, h, u, e represent complex numbers and * signifies acomplex conjugate. By executing the processing set forth above, thedistortion compensation coefficient h(p) is updated so as to minimizethe difference e(t) between the transmit signal x(t) and the feedback(demodulated) signal y(t), and the coefficient eventually converges tothe optimum distortion compensation coefficient h(p) so thatcompensation is made for the distortion in the transmission poweramplifier.

FIG. 51 is a diagram showing the overall construction of a transmittingapparatus expressed by x(t)=I(t)+jQ(t). Components in FIG. 51 identicalwith those shown in FIGS. 48 and 50 are designated by like referencecharacters.

As mentioned above, the principle of digital non-linear distortioncompensation is to feed back and detect a carrier obtained by quadraturemodulation of a transmit signal, digitally convert and compare theamplitudes of the transmit signal and feedback signal, and update thedistortion compensation coefficient based upon the comparison. Inaccordance with this method of digital non-linear distortioncompensation, distortion can be reduced. As a result, the ACPRrequirement is satisfied (i.e., leakage power can be held low) with ahigh output and even with operation in the non-linear region, and thepower added efficiency can be improved, thus making it possible toreduce power consumption. Further, the amount of heat evolved can bereduced by the improvement in power added efficiency, thereby mitigatingthe need for measures to deal with such heating. The end result is anapparatus of smaller size.

When distortion occurs, the signal undergoes amplitude distortion andphase distortion simultaneously. The reason for this is that when atransmit signal that has been compensated for distortion exceeds thecompensation amplitude limits of the distortion compensating circuit,the signal has its amplitude limited to the threshold value of thedistortion compensating apparatus, the amplitude value becomes stuck atthe upper-limit value of the distortion compensating apparatus and phasecontrol becomes impossible.

Though a transmission power amplifier has a non-linear characteristicowing to saturation, the amplifier is used in a state as close tosaturation as possible in view of transmission efficiency, as mentionedabove. On the other hand, since the distortion compensating apparatuscontrols distortion compensation in such a manner that thecharacteristic is made linear, the distortion compensation coefficienth_(n)(p) becomes gradually larger when the apparatus is used in a statenear saturation. As a consequence, there is a rise in the level of thetransmit signal x(t)*h(p) (where * signifies complex multiplication)after the distortion compensation thereof, the dynamic range of the DAconverter is exceeded and the amplitude of the output of the DAconverter becomes distorted. As a result, the transmit signal comes tocontain harmonic components, phase becomes distorted as well asamplitude and leakage between adjacent channels occurs. This means thatthe spectrum characteristic will be out of specs.

FIG. 52 is a diagram useful in describing problems encountered with theconventional phase compensating apparatus. The dashed line LM in FIG. 52indicates the dynamic range of the DA converter 3 (i.e., the DAconverter limit). Distortion will not occur if the level of the transmitsignal x(t)*h_(n)(p) output from the predistortion unit of thedistortion compensating apparatus is within the DA converter limit LM.However, if the distortion compensation coefficient h_(n+1)(p) for thetransmit signal x(t) increases owing to distortion compensationprocessing, then x(t)*h_(n+1)(p) will exceed the DA converter limit LM,the amplitude will be clamped to the DA converter limit LM, harmoniccomponents will be produced and phase will become distorted, asmentioned above.

More specifically, in the region where the power amplifier has a highdegree of non-linearity, the amplitude difference e(t) between thetransmit signal x(t) prior to correction and the feedback signalincreases without an increase in the amplitude of the feedback signaly(t) regardless of the fact that it is being attempted to enlarge theamplitude by distortion compensation. If the amplitude difference takeson a large value, the distortion compensator 8 judges that distortioncompensation is not being carried out as desired and enlarges thedistortion compensation coefficient h_(n+1)(p) in such a manner that thedifference signal e(t) becomes smaller. As a result, the signalamplitude after distortion compensation thereof is caused to increaseand, consequently, the signal amplitude exceeds the limit value (thelimit DM of the DA converter limit 3). This means that the amplitude ofa signal whose amplitude has exceeded the limit value takes on aconstant amplitude value, resulting in loss of significant components(amplitude and phase) of the signal. As a consequence, not onlycompensation of the amplitude component but also compensation of thephase component can no longer be carried out. In other words, anobstacle which arises is that distortion compensation does not operatenormally.

As a result of the foregoing, when amplitude exceeds the DA converterlimit LM, it becomes impossible to control both amplitude and phase andthe distortion characteristic becomes worse than that when no distortioncompensation is applied.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to correct beforehandthe size of a distortion compensation coefficient, while maintainingphase, in such a manner that the input amplitude of a DA converter willnot exceed the limit of the DA converter, thereby making it possible tofollow up phase by assuring that distortion will not occur even ifamplitude is limited.

Another object of the present invention is to correct beforehand thesize of a distortion compensation coefficient, while maintaining phase,in such a manner that the power of a transmit signal after itsdistortion compensation will not exceed an allowed upper-limit power,thereby making it possible to follow up phase even if amplitude islimited.

Another object of the present invention is to so arrange it that thecorrected value of a distortion compensation coefficient can becalculated in a simple manner.

A further object of the present invention is to reduce the differencebetween transmission power and a feedback signal by controlling theamplitude of the feedback signal, thereby preventing an increase in thedistortion compensation coefficient and assuring that the transmitsignal after its distortion compensation will not exceed a limit value.

Yet another object of the present invention is to make possibleapplication to (1) a distortion compensation method of multiplying atransmit signal by a distortion compensation coefficient and inputtingthe product to a distortion device, and (2) a distortion compensationmethod of generating, as an error signal, the difference between atransmit signal and a signal obtained by multiplying a referentialsignal (transmit signal) by a distortion compensation coefficient,DA-converting the error signal and a main signal (transmit signal)separately, combining the converted signals and inputting the result toa distortion device.

Still another object of the present invention is to provide a distortioncompensating apparatus that can be applied to a single-carriertransmitter and to a multicarrier transmitter.

In accordance with the present invention, the foregoing objects areattained by providing a distortion compensating apparatus in which adistortion compensation coefficient is corrected in advance and storedin a distortion compensation coefficient memory in such a manner that atransmit signal that has undergone distortion compensation processingwill not exceed the dynamic range of a DA converter. Specifically,before a distortion compensation coefficient h_(n+1)(p) is stored in adistortion compensation coefficient memory following the calculation ofthis coefficient, an assumption is made that distortion compensationwill be performed using the distortion compensation coefficienth_(n+1)(p). Then it is determined beforehand whether a signal that willbe obtained by this distortion compensation will exceed the limit of theDA converter. If the limit will be exceeded, the size of the distortioncompensation coefficient is reduced by a correction while the phasethereof is maintained, and the corrected distortion compensationcoefficient is stored in memory.

Further, in accordance with the present invention, the foregoing objectsare attained by determining, before a distortion compensationcoefficient h_(n+1)(p) is stored in a distortion compensationcoefficient memory following the calculation thereof, whether the power|x(t)*h_(n+1)(p)|² of a distortion-compensated signal x(t)*h_(n+1)(p) isgreater than a set upper-limit power Pmax of a DA converter and, if theupper-limit power is exceeded, reducing the size of thedistortion-compensated coefficient by a correction while the phasethereof is maintained and storing the corrected distortion-compensatedcoefficient in memory.

Further, in accordance with the present invention, the foregoing objectsare attained by determining, before a distortion compensationcoefficient h_(n+1)(p) is stored in a distortion compensationcoefficient memory following the calculation thereof, whether the squareof the distortion compensation coefficient h_(n+1)(p) is greater thanthe square of a set maximum distortion compensation coefficienth(p)_(MAX) and, if such is the case, reducing the size of the distortioncompensation coefficient by a correction while the phase thereof ismaintained and storing the corrected distortion compensation coefficientin memory. More specifically, if the arrangement described above isadopted, the distortion-compensated signal (the DA converter input) willno longer exceed the DA converter limit (dynamic range) and neitheramplitude nor phase distortion will occur in the DA converter. Moreover,since the size of the distortion compensation coefficient h_(n+1)(p) isreduced while the phase of this coefficient is maintained, it ispossible to follow up phase.

Further, in accordance with the present invention, the foregoing objectsare attained by controlling the amplitude of a feedback signal from theoutput of an amplifier when it is sensed that the amplitude of a signalafter distortion compensation has exceeded a certain limit. Byexercising control so as to enlarge the amplitude of the feedback signalwhen the limit is exceeded, the difference between a transmit signal andthe feedback signal is diminished, the distortion compensationcoefficient is reduced and it is possible to prevent the limit frombeing exceeded by the amplitude of a signal that has undergonedistortion compensation.

Further, in accordance with the present invention, the foregoing objectsare attained by putting the correspondence between the amplitude of atransmit signal or the power of the transmit signal and gain into theform of a table, obtaining from the table a gain that conforms to anactual transmit-signal amplitude or transmit-signal power, andcontrolling the amplitude of a feedback signal based upon this gain. Ifthis arrangement is adopted, the amplitude of a signal that hasundergone distortion compensation can be prevented from exceeding alimit value without sensing whether the amplitude of this signal hasexceeded the limit value.

It should be noted that the present invention is applicable to (1) afirst distortion compensation method of multiplying a transmit signal bya distortion compensation coefficient and inputting the product to adistortion device, and (2) a second distortion compensation method ofgenerating, as an error signal, the difference between a transmit signaland a signal obtained by multiplying a referential signal (the transmitsignal) by a distortion compensation coefficient, DA converting theerror signal and a main signal (transmit signal) separately, combiningthe converted signals and inputting the result to a distortion device.Furthermore, the present invention is applicable to a single-carriertransmitter and to a multicarrier transmitter.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the construction of asingle-carrier-type distortion compensating apparatus according to thepresent invention;

FIG. 2 is a diagram useful in describing the principles of the presentinvention;

FIG. 3 is a diagram showing the construction of a first embodiment ofthe present invention;

FIG. 4 is a diagram showing the construction of a second embodiment ofthe present invention;

FIG. 5 is a diagram showing the construction of a third embodiment ofthe present invention;

FIG. 6 is a diagram showing the construction of a fourth embodiment ofthe present invention;

FIG. 7 is a diagram showing the construction of a fifth embodiment ofthe present invention;

FIG. 8 is a diagram showing the construction of a sixth embodiment ofthe present invention;

FIG. 9 is a block diagram illustrating a multicarrier-type transmitterto which a distortion compensating apparatus has been added;

FIG. 10 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to aseventh embodiment;

FIG. 11 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to aneighth embodiment;

FIG. 12 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to a ninthembodiment;

FIG. 13 illustrates a first embodiment in which an analog transmitsignal and an error signal are combined and input to a distortiondevice;

FIG. 14 is a diagram useful in describing the principles of the presentinvention;

FIG. 15 illustrates a second embodiment in which an analog transmitsignal and an error signal are combined and input to a distortiondevice;

FIG. 16 illustrates a third embodiment in which an analog transmitsignal and an error signal are combined and input to a distortiondevice;

FIG. 17 illustrates a fourth embodiment in which an analog transmitsignal and an error signal are combined and input to a distortiondevice;

FIG. 18 is a block diagram illustrating a multicarrier-type transmitterto which a distortion compensating apparatus has been added;

FIG. 19 is a diagram useful in describing a frequency conversion;

FIG. 20 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to a fifthembodiment;

FIG. 21 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to a sixthembodiment;

FIG. 22 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to aseventh embodiment;

FIG. 23 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to aneighth embodiment;

FIG. 24 is a block diagram illustrating another multicarrier-typetransmitter to which a distortion compensating apparatus has been added;

FIG. 25 is a diagram useful in describing a frequency conversion;

FIG. 26 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to a ninthembodiment;

FIG. 27 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to a tenthembodiment;

FIG. 28 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to an 11thembodiment;

FIG. 29 is a block diagram illustrating the construction of amulticarrier-type distortion compensating apparatus according to a 12thembodiment;

FIG. 30 illustrates a first embodiment of a distortion compensatingapparatus having a function for controlling the amplitude of a feedbacksignal;

FIGS. 31A to 31C are diagrams useful in describing the relationshipbetween transmit-signal amplitude (power) and gain;

FIG. 32 illustrates a first modification of the first embodiment;

FIG. 33 illustrates a second modification of the first embodiment;

FIG. 34 illustrates a second embodiment of a distortion compensatingapparatus having a function for controlling the amplitude of a feedbacksignal;

FIG. 35 illustrates a third embodiment of a distortion compensatingapparatus having a function for controlling the amplitude of a feedbacksignal;

FIG. 36 illustrates a fourth embodiment of a distortion compensatingapparatus having a function for controlling the amplitude of a feedbacksignal;

FIG. 37 illustrates the overall processing flow of the fourthembodiment;

FIG. 38 illustrates a first modification of the fourth embodiment;

FIG. 39 illustrates a second modification of the fourth embodiment;

FIG. 40 is a diagram useful in describing the relationship between a μvalue and convergence time;

FIG. 41 is a diagram illustrating an example of an arrangement in whicha multicarrier-type transmitter is equipped with a distortioncompensating apparatus according to the present invention;

FIG. 42 is illustrates an example of the effects of a multicarrierlinearizer;

FIG. 43 illustrates a sixth embodiment of a distortion compensatingapparatus having a function for controlling the amplitude of a feedbacksignal;

FIG. 44 is a diagram illustrating the construction of amulticarrier-type transmitter;

FIG. 45 is a block diagram showing a transmitter according to the priorart;

FIGS. 46A and 46B are diagrams useful in describing problems which ariseowing to the non-linearity of a transmission power amplifier accordingto the prior art;

FIG. 47 is a diagram useful in describing the efficiency characteristicof a power amplifier according to the prior art;

FIG. 48 is a block diagram of a transmitter equipped with a digitalnon-linear distortion compensating apparatus according to the prior art;

FIG. 49 is a diagram illustrating the function of a compensatoraccording to the prior art;

FIG. 50 is a diagram useful in describing distortion compensationprocessing according to an adaptive LMS algorithm;

FIG. 51 is a diagram showing the overall construction of a transmitterexpressed by x(t)=I(t)+jQ(t) according to the prior art; and

FIG. 52 is a diagram useful in describing the problems that arise with adistortion compensating apparatus according to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Principles of the PresentInvention

(a) Overview

FIG. 1 is a block diagram illustrating an overview of the presentinvention. The apparatus includes a device (a transmission poweramplifier) 21 which produces non-linear distortion of a function f(p); adistortion compensation coefficient memory 22 for storing a distortioncompensation coefficient h(p), which corrects the distortion of thetransmission power amplifier 21, in association with power p (=|x(t)|²)of a transmit signal x(t); a predistortion unit 23 for reading adistortion compensation coefficient h_(n)(p) conforming to the power pof the transmit signal x(t) out of the memory 22 and applying distortioncompensation processing to the transmit signal using this distortioncompensation coefficient; a DA converter 24 for converting a digitaltransmit signal x(t)*h_(n)(p), which is the result of the applieddistortion compensation processing, to an analog signal; a feedback loop25 for feeding back the output signal y(t) of the transmission poweramplifier 21; an AD converter 26 for converting the feedback signal todigital data; a distortion compensation coefficient calculation unit 27for calculating a distortion compensation coefficient h_(n+1)(p) basedupon the transmit signal x(t) before the distortion compensation thereofand the feedback signal; a distortion compensation coefficient updatingunit 28 for updating the distortion compensation coefficient by storingthe calculated distortion compensation coefficient h_(n+1)(p) or acorrected distortion compensation coefficient h_(n+1)(p)′ in associationwith the power |x(t)|² of the transmit signal x(t); and a comparisonunit 29. Before the distortion compensation coefficient h_(n+1)(p)calculated by the distortion compensation coefficient calculation unit27 is stored in the memory 22, the comparison unit 29 compares powerPa[=|x(t)*h_(n+1)(p)|²] of a transmit signal that would be output fromthe predistortion unit 23 owing to distortion compensation coefficientprocessing that is based upon the distortion compensation coefficienth_(n+1)(p), with maximum power Pmax stipulated by the dynamic range ofthe DA converter 24. The apparatus further includes a distortioncompensation coefficient correction unit 30 and a transmit-signal powercalculation unit 31 for generating read/write addresses of thedistortion compensation coefficient memory 22.

(b) Principles of the Present Invention

FIG. 2 is a diagram useful in describing the principles of the presentinvention.

In the digitally implemented distortion compensating apparatus of FIG.1, the upper limit of the distortion-compensated signal is stipulated bythe number of bits in the digital data or the number of bits (thedynamic range) of the DA converter 24. It will be assumed below that theupper limit is decided by the dynamic range of the DA converter 24. InFIG. 2, a small square LM_(S) indicated by the dashed line is the DAconverter limit decided by the dynamic range of the DA converter, and alarge square LM_(L) indicated by the dashed line is a calculation limitdecided by the number of bits in the digital data. Let x(t) represent atransmit signal before it is subjected to distortion compensationprocessing, and let h_(n)(p) represent a distortion compensationcoefficient that conforms to the power of this transmit signal. Thedistortion-compensated signal output from the predistortion unit 23 isx(t)*h_(n)p. If the distortion-compensated signal x(t)*h_(n)p fallswithin a circle tangent to the DA converter limit LM_(S), the transmitsignal x(t) will not exceed the DA converter limit LM_(L) regardless ofthe phase and neither amplitude nor phase will be distorted in the DAconverter 24.

However, when the output amplitude of the transmission power amplifier21 rises owing to the action of the distortion function f(p), thedifference between the transmit signal x(t) before the distortioncompensation thereof and the feedback signal increases and thedistortion compensation coefficient h_(n+1)(p) output from thedistortion compensation coefficient calculation unit 27 grows larger. Ifin such case the distortion compensation coefficient h_(n+1)(p) isstored in the distortion compensation coefficient memory 22 as iswithout being corrected, this distortion compensation coefficient willbe read out of the memory 22 and a distortion-compensated signalx(t)*h_(n+1)(p) will be output from the predistortion unit 23. If thedistortion-compensated signal x(t)*h_(n+1)(p) exceeds the DA converterlimit LM_(S), amplitude distortion and phase distortion will occur inthe DA converter.

Accordingly, before the distortion compensation coefficient h_(n+1)(p)is stored in the memory 22 following the calculation of thiscoefficient, an assumption is made that distortion compensation will beperformed using the distortion compensation coefficient h_(n+1)(p). Thenit is determined beforehand whether a signal that will be obtained bythis distortion compensation will exceed the limit of the DA converter.If it is judged that the limit will be exceeded, this distortioncompensation coefficient is corrected so as to be reduced in size whilethe phase thereof is maintained as is. If this arrangement is adopted,the input to the DA converter will be limited, the DA converter limitLM_(s) will not be exceeded and neither amplitude nor phase distortionwill occur in the DA converter. Moreover, phase follow-up becomespossible because the size of the distortion compensation coefficienth_(n+1)(p) is reduced while the phase thereof is maintained.

(c) Amplitude Limitation by Amplitude Control

It will be understood from the foregoing that when the distortioncompensation coefficient h_(n+1)(p) has been found, the comparison unit29 compares the distortion-compensated signal x(t)*h_(n+1)(p) that wouldbe output from the predistortion unit 23 owing to distortioncompensation coefficient processing that is based upon the distortioncompensation coefficient h_(n+1)(p), with the DA converter limit LM_(s)before the distortion compensation coefficient h_(n+1)(p) is stored inthe memory 22. Further, the distortion compensation coefficientcorrection unit 30 corrects the distortion compensation coefficienth_(n+1)(p) to h_(n+1)(p)/m so that x(t)*h_(n+1)(p) will become smallerthan the DA converter limit LM_(s). If the distortion-compensated signalx(t)*h_(n+1)(p) is greater than the DA converter limit LM_(s), then thedistortion compensation coefficient updating unit 28 stores thecorrected distortion compensation coefficient h_(n+1)(p)/m in thedistortion compensation coefficient memory 22. If thedistortion-compensated signal x(t)*h_(n+1)(p) is less than the DAconverter limit LM_(s), then the distortion compensation coefficientupdating unit 28 stores the uncorrected distortion compensationcoefficient h_(n+1)(p) in the distortion compensation coefficient memory22 as is. It should be noted that x(t)*h_(n+1)(p) is a complex number.In general, therefore, amplitude control usually is carried out basedupon power in the manner described in (d) below.

(d) Amplitude Limitation by Power Control

In (c) above, the amplitude of the input signal to the DA converter iscontrolled directly and control is exercised in such a manner that theamplitude will converge to the DA converter limit LM_(s). However,control can be carried out in such a manner that the power|x(t)*h_(n+1)(p)|² of the distortion-compensated signal x(t)*h_(n+1)(p)will fall below the upper-limit power Pmax of the DA converter, wherebythe amplitude will fall within an envelope circle ECIR that is tangentto the DA converter limit LM_(s). In this case, when the distortioncompensation coefficient h_(n+1)(p) has been found, the comparison unit29 compares the power Pa[=|x(t)*h_(n+1)(p)|²] of the distortioncompensation coefficient that would be output from the predistortionunit 23 owing to distortion compensation coefficient processing that isbased upon the distortion compensation coefficient h_(n+1)(p), with theupper-limit power Pmax of the DA converter 24 before the distortioncompensation coefficient h_(n+1)(p) is stored in the memory 22. Further,when |x(t)*h_(n+1)(p)|² is greater than the upper-limit power Pmax, thedistortion compensation coefficient correction unit 30 corrects thedistortion compensation coefficient h_(n+1)(p) to h_(n+1)(p)/m so thatthe upper-limit power will not be exceeded. When the power Pa of thedistortion-compensated signal is greater than the upper-limit powerPmax, the distortion compensation coefficient updating unit 28 storesthe corrected distortion compensation coefficienth_(n+1)(p)′(=h_(n+1)(p)/m) in the distortion compensation coefficientmemory 22. When the power Pa is less than the upper-limit power Pmax,the distortion compensation coefficient h_(n+1)(p) is not corrected andis stored in the distortion compensation coefficient memory 22 as is.

As a result of the foregoing, a distortion-compensated signalx(t)*h_(n+1)(p)/m that has been distortion-compensated by the distortioncompensation coefficient h_(n+1)(p)′ falls within the envelope circleECIR and neither amplitude distortion nor phase distortion occur in theDA converter 24. In other words, though compensation for amplitudedistortion is not perfect, phase is followed up. This means that thedistortion characteristic will not be worse than the distortioncharacteristic that would be obtained without application of thedistortion compensating apparatus.

(e) Alternative Amplitude Limitation by Power Control

If we let Pmax represent the allowable upper-limit power set for the DAconverter and let h(p)_(MAX) represent the maximum distortioncompensation coefficient for the transmit signal x(t), then thefollowing relation will hold:Pmax=|x(t)*h(p)_(MAX)|²Since Pmax is constant, the transmit signal x(t) and the maximumdistortion compensation coefficient h(p)_(MAX) have a 1:1 relationship.If the transmit signal x(t) has been determined, then the maximumdistortion compensation coefficient h(p)_(MAX) will be uniquely decided.Accordingly, if the square of the distortion compensation coefficienth_(n+1)(p) is greater than the square of the maximum distortioncompensation coefficient h(p)_(MAX), the distortion-compensated signalx(t)*h_(n+1)(p) that will be obtained using the distortion compensationcoefficient h_(n+1)(p) will exceed the envelope circle ECIR.

It will be understood from the foregoing that when the distortioncompensation coefficient h_(n+1)(p) has been calculated, the comparisonunit 29 compares the square of the distortion compensation coefficient(namely |h_(n+1)(p)|²) and the square of the maximum distortioncompensation coefficient h(p)_(MAX) (namely |h(p)_(MAX)|²). Thedistortion compensation coefficient correction unit 30 corrects thedistortion compensation coefficient h_(n+1)(p) to h_(n+1)(p)/m so that|h_(n+1)(p)|² will become smaller than |h(p)_(MAX)|². When|h_(n+1)(p)|²>|h(p)_(MAX)|² holds, the distortion compensationcoefficient updating unit 28 stores the corrected distortioncompensation coefficient h_(n+1)(p)′[=h_(n+1)(p)/m] in the distortioncompensation coefficient memory 22. When |h_(n+1)(p)|²<|h(p)_(MAX)|²holds, the distortion compensation coefficient updating unit 28 does notcorrect the distortion compensation coefficient h_(n+1)(p) and stores itin the distortion compensation coefficient memory 22 as is. As a result,a signal x(t)*h_(n+1)(p)/m that has been distortion-compensated by thedistortion compensation coefficient h_(n+1)(p)′ falls within theenvelope circle ECIR and neither amplitude distortion nor phasedistortion occur in the DA converter 24.

(f) Implementation not Requiring Calculation for Correcting DistortionCompensation Coefficients

If corrected values of distortion compensation coefficients are storedin association with combinations of |x(t)|² and h_(n+1)(p), it will notbe necessary to execute processing such as calculation for correctingdistortion compensation coefficients and comparison of the power Pa ofthe distortion-compensated signal and the upper-limit power Pmax. Hence,(1) the distortion compensation coefficient h_(n+1)(p) is corrected insuch a manner that the power Pa of the transmit signal when distortioncompensation processing is applied to the transmit signal x(t) using thedistortion compensation coefficient h_(n+1)(p) calculated by thedistortion compensation coefficient calculation unit will fall below theupper-limit power Pmax, and (2) the corrected distortion compensationcoefficient h_(n+1)(p)′ is put in a table in association with thecombination of |x(t)|² and h_(n+1)(p). (3) When the power Pa of thedistortion-compensated transmit signal is less than the upper-limitpower Pmax, the distortion compensation coefficient h_(n+1)(p) is put ina table as is in association with the combination of |x(t)|² andh_(n+1)(p).

If the distortion compensation coefficient h_(n+1)(p) is calculated bythe distortion compensation coefficient calculation unit 27 under theseconditions, the distortion compensation coefficient updating unit 28obtains from the table the corrected value of the distortioncompensation coefficient that conforms to the combination of thedistortion compensation coefficient h_(n+1)(p) and the power |x(t)|² ofthe transmit signal x(t) and stores this corrected value in thedistortion compensation coefficient memory 22. When distortioncompensation processing for the next transmit signal x(t) issubsequently executed, the predistortion unit 23 reads the correcteddistortion compensation coefficient out of the memory 22, executesdistortion compensation processing and outputs the result.

The foregoing is an example in which a distortion compensationcoefficient is corrected using a table before a coefficient is writtento the memory 22. However, an arrangement in which a coefficient is readout of the memory 22 and is corrected using the above-mentioned tablecan be adopted. Specifically, when the distortion compensationcoefficient h_(n+1)(p) has been calculated by the distortioncompensation coefficient calculation unit 27, the distortioncompensation coefficient updating unit 28 stores the distortioncompensation coefficient h_(n+1)(p) in the distortion compensationcoefficient memory 22 as is. Then, when the transmit signal x(t) is tobe subjected to distortion compensation processing, the distortioncompensation coefficient h_(n)(p) is read out of the memory 22. At thistime the predistortion unit 23 obtains from the table the correctedvalue of the distortion compensation coefficient conforming to thecombination of the distortion compensation coefficient h_(n)(p) andpower |x(t)|² of the transmit signal x(t), executes distortioncompensation processing and outputs the result.

(g) Amplitude Limitation by Amplitude Control of Feedback Signal

When it has been sensed that the amplitude of a signal following thedistortion compensation thereof is over the limit, the amplitude of thefeedback signal from the output of the amplifier is raised. By raisingthe amplitude of the feedback signal when the limit has been exceeded,the difference between the transmit signal and the feedback signal isreduced and the distortion compensation coefficient decreases so that itis possible to prevent the amplitude of the compensation-limited signalfrom subsequently surpassing the limit.

Further, correspondence between the amplitude or power of the transmitsignal and gain is put into table form, gain that conforms to theamplitude or power of an actual transmit signal is found from the table,and the amplitude of the feedback signal is controlled based upon thegain found. If this arrangement is adopted, the amplitude of a signalthat has undergone distortion compensation can be prevented fromexceeding a limit value without sensing whether the amplitude of thissignal has exceeded the limit value.

(B) Embodiments of Distortion Compensating Apparatus for Outputting aDistortion Compensation Signal by Multiplying a Transmit Signal by aDistortion Compensation Coefficient (a) First Embodiment

FIG. 3 illustrates a first embodiment of the present invention appliedto a distortion compensating apparatus for outputting adistortion-compensated signal by multiplying a transmit signal by adistortion compensation coefficient. Components in FIG. 3 identical withthose illustrated in FIG. 1 are designated by like reference characters.Numerals 32 to 37 denote delay circuits for adjusting timing.

The distortion compensation coefficient calculation unit 27 calculates adistortion compensation coefficient h_(n+1)(n) by an LMS algorithm in amanner similar to that of the example of the prior art shown in FIG. 34.The distortion compensation coefficient calculation unit 27 includes acomplex-conjugate signal output unit 27 a; a subtractor 27 b foroutputting the difference e(t) between the transmit signal x(t) prior toits distortion compensation processing and a feedback (demodulated)signal y(t); a multiplier 27 c for multiplying e(t) and u*(t); amultiplier 27 d for multiplying h_(n)(p) and y*(t) a multiplier 27 e formultiplying the output of the multiplier 27 c by a step-size parameterμ; and an adder 27 f for adding h_(n)(p) and μe(t)u*(t).

The distortion compensation coefficient updating unit 28 updates thedistortion compensation coefficient stored in the distortioncompensation coefficient memory 22 and has a selector 28 a. The latterstores a distortion compensation coefficient X[=h_(n+1)(p)/m] in thedistortion compensation coefficient memory 22 when the powerPa[=|x(t)*h_(n+1)(p)|²] of the distortion-compensated signal is largerthan the upper-limit power Pmax set beforehand in dependence upon thedynamic range of the DA converter 24, and sets an uncorrected distortioncompensation coefficient Y[=h_(n+1)(p)] in the distortion compensationcoefficient memory 22 when the power Pa is less than Pmax.

The comparison unit 29 compares the power Pa[=|x(t)*h_(n+1)(p)|²] of thedistortion-compensated signal with the set upper-limit power Pmax andincludes a power calculation unit 29 a and a comparator 29 b. The powercalculation unit 29 a calculates the power Pa of thedistortion-compensated signal x(t)*h_(n+1)(p) output from thepredistortion unit 23 by distortion compensation processing using thedistortion compensation coefficient h_(n+1)(p) obtained by thedistortion compensation coefficient calculation unit 27, and thecomparator 29 b compares the power Pa[=|x(t)*h_(n+1)(p)|²] and theupper-limit power Pmax and inputs the result to the selector 28 a. Itshould be noted that * signifies complex multiplication.

When the power Pa[=|x(t)*h_(n+1)(p)|²] of the transmit signal is greaterthan the upper-limit power Pmax, the distortion compensation coefficientcorrection unit 30 corrects the distortion compensation coefficienth_(n+1)(p) to h_(n+1)(p)/m so that Pa will fall below the upper-limitpower Pmax. The distortion compensation coefficient correction unit 30has an m-value controller 30 a and a corrected-value calculation unit 30b. Here m represents a coefficient attenuation ratio and m² is the ratioof the power Pa[=|x(t)*h_(n+1)(p)|²] of the distortion-compensatedsignal to the upper-limit power Pmax. Accordingly, the followingrelation holds:m ² =|x(t)*h _(n+1)(p)|² /Pmaxand m is found fromm={|x(t)*h _(n+1)(p)|² /Pmax}^(1/2)  (1)The m-value controller 30 a performs the calculation of Equation (1) tocalculate the coefficient attenuation ratio m, and the corrected-valuecalculation unit 30 b outputs the corrected value X of the distortioncompensation coefficient upon calculating the same in accordance withthe following equation:X=h _(n+1)(p)/m  (2)

The delay circuit 32 delays the output signal from the transmit-signalpower calculation unit 31 in such a manner that a write address (Writeadr) is generated at the timing at which the distortion compensationcoefficient is output from the selector 28 a. The delay circuit 33matches the timing of the transmit signal x(t) before the distortioncompensation thereof to the timing of the feedback signal y(t). Thetransmit signal is delayed until the feedback signal arrives at thesubtractor 27 b. The delay circuit 34 adjusts the timing of the inputsignal to the adder 27 f. Specifically, the delay circuit 34 delays thedistortion compensation coefficient h_(n)(p), which is output from thedistortion compensation coefficient memory 22, until μe(t)u*(t) isoutput from the multiplier 27 e. The delay circuit 35 adjusts the timingof the input signal to the predistortion unit 23. Specifically, thedelay circuit 35 delays the transmit signal x(t) for a length of timefrom the moment the distortion compensation coefficient is read out ofthe distortion compensation coefficient memory 22 to the moment thisdistortion compensation coefficient enters the predistortion unit 23.The delay circuit 36 adjusts the timing of the input signal to the powercalculation unit 29 a. That is, the delay circuit 36 delays the transmitsignal x(t) until the distortion compensation coefficient h_(n+1) isoutput from the distortion compensation coefficient calculation unit 27.The delay circuit 37 matches the output timings of the selector inputs Xand Y, i.e., delays the output timing of the distortion compensationcoefficient h_(n+1) until the corrected value X is generated.

When the distortion compensation coefficient h_(n+1)(p) has been found,the comparison unit 29 compares the power |x(t)*h_(n+1)(p)|² of thedistortion compensation coefficient that would be output from thepredistortion unit 23 owing to distortion compensation coefficientprocessing that is based upon the distortion compensation coefficienth_(n+1)(p), with the upper-limit power Pmax of the DA converter 24.Further, the distortion compensation coefficient correction unit 30corrects the distortion compensation coefficient h_(n+1)(p) toh_(n+1)(p)/m in such a manner that the power |x(t)*h_(n+1)(p)|² willfall below the upper-limit power Pmax. When the power Pa of thedistortion-compensated signal is greater than the upper-limit powerPmax, the distortion compensation coefficient updating unit 28 storesthe corrected value X (=h_(n+1)(p)/m) of the distortion compensationcoefficient in the distortion compensation coefficient memory 22. Whenthe power Pa is less than the upper-limit power Pmax, the uncorrecteddistortion compensation coefficient Y[=h_(n+1)(p)] is stored in thedistortion compensation coefficient memory 22. When distortioncompensation processing for the next transmit signal x(t) issubsequently executed, the predistortion unit 23 reads the correcteddistortion compensation coefficient out of the memory 22, executesdistortion compensation processing and outputs the result. Now thedistortion-compensated signal will fall within the envelope circle ECIR(see FIG. 2) and neither amplitude nor phase distortion will occur inthe DA converter.

(b) Second Embodiment

FIG. 4 illustrates a second embodiment of the present invention appliedto a distortion compensating apparatus for outputting adistortion-compensated signal by multiplying a transmit signal by adistortion compensation coefficient. Components in FIG. 4 identical withthose illustrated in FIG. 3 are designated by like reference characters.This embodiment differs in terms of the construction of the comparisonunit 29, which here includes a table 29 c, an arithmetic unit 29 d and acomparator 29 e. The table 29 c stores the square |h(p)_(MAX)|² of themaximum distortion compensation coefficient h(p)_(MAX) that conforms tothe power |x(t)|² of the transmit signal x(t), the arithmetic unit 29 dcalculates the square |h_(n+1)(p)|² of the distortion compensationcoefficient h_(n+1)(p), and the comparator 29 e compares |h(p)_(MAX)|²and |h_(n+1)(p)|².

If we let Pmax represent the allowable upper-limit power set for the DAconverter and let h(p)_(MAX) represent the maximum distortioncompensation coefficient for the transmit signal x(t), then thefollowing relation will hold:Pmax=|x(t)*h(p)_(MAX)|²  (3)Since Pmax is constant, the transmit signal x(t) and the maximumdistortion compensation coefficient h(p)_(MAX) have a 1:1 relationship.If the transmit signal x(t) has been determined, then the maximumdistortion compensation coefficient h(p)_(MAX) will be uniquely decided.Accordingly, the size relationship between Pmax[=|x(t)*h(p)_(MAX)|²] andthe power Pa[=|x(t)*h_(n+1)(p)|²] of the distortion-compensated signalagrees with the size relationship between |h(p)_(MAX)|² and|h_(n+1)(p)|². The distortion compensation coefficient correction unit30 corrects the distortion compensation coefficient h_(n+1)(p) toh_(n+1)(p)/m in accordance with Equations (1), (2) so that |h_(n+1)(p)|²will become smaller than |h(p)_(MAX)|². When |h_(n+1)(p)|²>|h(p)_(MAX)|²holds, the distortion compensation coefficient updating unit 28 storesthe corrected distortion compensation coefficient X[=h_(n+1)(p)/m] inthe distortion compensation coefficient memory 22. When|h_(n+1)(p)|²<|h(p)_(MAX)|² holds, the distortion compensationcoefficient updating unit 28 stores the uncorrected distortioncompensation coefficient Y[=h_(n+1)(p)] in the distortion compensationcoefficient memory 22 as is. When distortion compensation processing forthe next transmit signal x(t) is subsequently executed, thepredistortion unit 23 reads the corrected distortion compensationcoefficient out of the memory 22, executes distortion compensationprocessing and outputs the result. Now the distortion-compensated signalwill fall within the envelope circle ECIR (see FIG. 2) and neitheramplitude nor phase distortion will occur in the DA converter.

(c) Third Embodiment

FIG. 5 illustrates a third embodiment of the present invention appliedto a distortion compensating apparatus for outputting adistortion-compensated signal by multiplying a transmit signal by adistortion compensation coefficient. Components in FIG. 5 identical withthose illustrated in FIG. 3 are designated by like reference characters.This embodiment differs in the following respects:

(1) a corrected value h_(n+1)(p)′ of the distortion compensationcoefficient h_(n+1)(p) is stored in a distortion compensation valuelimiter table 41 beforehand in association with a combination of |x(t)|²and h_(n+1)(p);

(2) the corrected value of a desired distortion compensation coefficientis read out of the table 41 and stored in the distortion compensationcoefficient memory 22; and

(3) the comparison unit 29 and distortion compensation coefficientcorrection unit 30 are deleted.

The distortion compensation value limiter table 41 is created asfollows: The distortion compensation coefficient h_(n+1)(p) is correctedto h_(n+1)(p)/m so that the power |x(t)*h_(n+1)(p)|² of thedistortion-compensated signal obtained when distortion compensationprocessing is applied to the transmit signal x(t) using the distortioncompensation coefficient h_(n+1)(p) will fall below the upper-limitpower Pmax. The corrected value h_(n+1)(p)′[=h_(n+1)(p)/m] is then putinto table form in correspondence with the combination of |x(t)|² andh_(n+1)(p). In this case, if |x(t)*h_(n+1)(p)|² is smaller than theupper-limit power Pmax, the distortion compensation coefficienth_(n+1)(p) is not corrected and is put into table form in associationwith the combination of |x(t)|² and h_(n+1)(p)

When the distortion compensation coefficient calculation unit 27calculates the distortion compensation coefficient h_(n+1)(p) indistortion compensation processing, the distortion compensationcoefficient updating unit 28 obtains, from table 41, the corrected valueh_(n+1)(p)′ of the distortion compensation coefficient conforming to thecombination of the distortion compensation coefficient h_(n+1)(p) andpower |x(t)|² of the transmit signal x(t) and stores this value in thedistortion compensation coefficient memory 22. When distortioncompensation processing for the next transmit signal x(t) issubsequently executed, the predistortion unit 23 reads the correcteddistortion compensation coefficient out of the memory 22, executesdistortion compensation processing and outputs the result.

If above-described arrangement is adopted, comparison andcorrected-value calculation operations can be dispensed with and acorrected value of a distortion compensation coefficient can be obtainedat high speed. The arrangement is simpler as well.

(d) Fourth Embodiment

FIG. 6 illustrates a fourth embodiment of the present invention appliedto a distortion compensating apparatus for outputting adistortion-compensated signal by multiplying a transmit signal by adistortion compensation coefficient. Components in FIG. 6 identical withthose illustrated in FIG. 5 are designated by like reference characters.This embodiment differs in the location of the table 41. In the thirdembodiment, the table 41 is provided on the input side of the distortioncompensation coefficient memory 22. In the fourth embodiment, however,the table 41 is provided on the output side of the distortioncompensation coefficient memory 22. In the fourth embodiment, in otherwords, the distortion compensation coefficient is corrected using thetable 41 after the distortion compensation coefficient is read out ofthe memory 22.

When the distortion compensation coefficient calculation unit 27calculates the distortion compensation coefficient h_(n+1)(p) indistortion compensation processing, the distortion compensationcoefficient updating unit 28 stores the distortion compensationcoefficient h_(n+1)(p) in the distortion compensation coefficient memory22 as is. When distortion compensation processing for the next transmitsignal x(t) is executed and the distortion compensation coefficienth_(n)(p) is read out of the memory 22, the predistortion unit 23obtains, from table 41, the corrected distortion compensationcoefficient h_(n)(p)′ that conforms to the combination of the distortioncompensation coefficient h_(n)(p) and |x(t)|², executes distortioncompensation processing and outputs the result.

(e) Fifth Embodiment

FIG. 7 illustrates a fifth embodiment in which the operation forcorrecting a distortion compensation coefficient is simplified.Components in FIG. 7 identical with those of the first embodiment inFIG. 3 are designated by like reference characters. In the firstembodiment, the operation for correcting a distortion compensationcoefficient requires calculation and this in turn results in hardware oflarger scale. In the fifth embodiment, the calculation is achieved by abit shift. According to the fifth embodiment, when the power|x(t)*h_(n+1)(p)|² of the distortion-compensated signal is greater thanthe upper-limit power Pmax, the distortion compensation coefficienth_(n+1)(p) is corrected to [h_(n+1)(p)−h_(n+1)(p)/r] so that thefollowing equations will hold:[{h _(n)(p)+Δh _(n+1)(p)}−{h _(n)(p)+Δh _(n+1)(p)}/r] ²≦(h_(n)(p))²  (4)[h _(n+1)(p)−h _(n+1)(p)/r] ² ≦[h _(n)(p)]²  (4)′In the above equations, Δh_(n+1)(p) represents the output of themultiplier 27 e. If r is decided in such a manner that Equation (4)′ isdecided and the corrected value h_(n+1)(p)′ is expressed byh _(n+1)(p)′=[h _(n+1)(p)−h _(n+1)(p)/r]  (5)then h_(n)(p) will be less than the DA converter limit. The correctedvalue h_(n+1)(p)′, therefore, will also fall below the limit value withcertainty.

If Equation (4) is transformed, we have[h _(n)(p)+{Δh _(n+1)(p)−(h _(n)(p)+Δh _(n+1)(p))/r}] ²≦(h_(n)(p))²  (4)″If the following relation holds:Δh _(n+1)(p)−(h _(n)(p)+Δh _(n+1)(p))/r≦0  (6)then Equation (4) will hold true without fail. Accordingly, it willsuffice to decide r so as to satisfy the following equation on the basisof Equation (6):r≦{h _(n)(p)+Δh _(n+1)(p)}/Δh _(n+1)(p)=h _(n+1)(p)/Δh _(n+1)(p)  (6)′and to correct the distortion compensation coefficient h_(n+1)(p) inaccordance with Equation (5). Equation (6)′, however, involvescalculation. Accordingly, r is found by obtaining the largest integer Hthat satisfies2^(H) ≦Δh _(n+1)(p)and shifting h_(n+1)(p) a total of H times. In this case, Equation (5)involves calculation. Accordingly, if the smallest integer R thatsatisfiesr≦2Ris found, then the right side of Equation (5) can be obtained in simplefashion by shifting the distortion compensation coefficient h_(n+1)(p),which is the numerator, R times.

In the distortion compensation coefficient correction unit 30, anr-value controller 30 c obtains and outputs the smallest integral valueR that satisfies Equation (7), and an arithmetic unit 30 d performs theoperationh _(n+1)(p)′=[h _(n+1)(p)−h _(n+1)(p)/r]and inputs the result to the selector 28 a. The latter stores thecorrected value X (=[h_(n+1)(p)−h_(n+1)(p)/r]) in the distortioncompensation coefficient memory 22 when the power Pa of thedistortion-compensated signal is greater than the upper-limit powerPmax, and stores the uncorrected distortion compensation coefficientY[=h_(n+1)(p)] in the distortion compensation coefficient memory 22 ifPa is less than Pmax.

(f) Sixth Embodiment

FIG. 8 illustrates a sixth embodiment in which the operation forcorrecting a distortion compensation coefficient is simplified.Components in FIG. 8 identical with those of the second embodiment inFIG. 4 are designated by like reference characters. In the secondembodiment, the operation for correcting a distortion compensationcoefficient requires calculation and this in turn results in hardware oflarger scale. In the sixth embodiment, the calculation is achieved by abit shift. Though the construction of the distortion compensationcoefficient correction unit 30 is different from that of the secondembodiment, it is exactly the same as that of FIG. 7.

(g) Seventh Embodiment

The first to sixth embodiments are examples in which the invention isapplied to a single-carrier transmitter. However, the invention can beapplied to a multicarrier transmitter as well. FIG. 9 is a block diagramshowing the construction of a transmitter in a case where a plurality oftransmit signals are transmitted using a multicarrier signal. Thisillustrates an example of a case where four frequencies are multiplexedand transmitted. Digital transmit signals x₁(t), x₂(t), x₃(t), x₄(t) aremultiplied by exp(jω₁t), exp(jω₂t), exp(jω₃t), exp(jω₄t)(ω_(n)=2πf_(n)), respectively, by frequency shifters 51, 52, 53, 54,respectively, to effect a frequency shift to frequencies f₁, f₂, f₃, f₄,after which these frequencies are frequency-multiplexed by a combiner55. The digital frequency-multiplexed signal corresponds to thesingle-carrier transmit signal and subsequently undergoes distortioncompensation processing similar to the processing executed in the caseof the single carrier.

FIG. 10 illustrates a seventh embodiment in which the distortioncompensating apparatus of the first, second, fifth and sixth embodimentsis applied to the multicarrier transmitter of FIG. 9. Components in FIG.10 identical with those of the foregoing embodiments are designated bylike reference characters.

FIG. 11 illustrates an eighth embodiment in which the distortioncompensating apparatus of the third embodiment is applied to themulticarrier transmitter of FIG. 9. Components in FIG. 11 identical withthose of FIGS. 5 and 9 are designated by like reference characters.

FIG. 12 illustrates a ninth embodiment in which the distortioncompensating apparatus of the fourth embodiment is applied to themulticarrier transmitter of FIG. 9. Components in FIG. 12 identical withthose of FIGS. 6 and 9 are designated by like reference characters.

(C) Distortion Compensating Apparatus for Combining an Error Signal witha Main Signal and Inputting the Result to a Distortion Device

The foregoing embodiments are examples wherein the invention is appliedto a distortion compensating apparatus in which a transmit signal ismultiplied by a distortion compensation coefficient to generate adistortion-compensated signal and the latter is input to a transmissionpower amplifier. However, the invention is applicable also to adistortion compensating apparatus in which a main signal (transmitsignal) and a distortion component (error signal) appended to thetransmit signal are DA-converted independently of each other and thencombined and input to the transmission power amplifier. In accordancewith the latter distortion compensating apparatus, the amplitude of theerror signal is small and therefore the bit precision of a DA converter,which outputs only the error signal, can be reduced. Further, a DAconverter that outputs only the transmit signal need not have a largedynamic range and therefore the bit precision of this DA converter canbe reduced. These are the advantages of this distortion compensatingapparatus.

(a) First Embodiment

FIG. 13 illustrates a first embodiment of a distortion compensatingapparatus for combining an analog transmit signal and an error signal.Components in FIG. 13 identical with those of the embodiments thus farare designated by like reference characters.

As shown in FIG. 13, the apparatus includes the device (the transmissionpower amplifier) 21 which produces non-linear distortion of a functionf(p); the distortion compensation coefficient memory 22 for storing thedistortion compensation coefficient h(p), which corrects the distortionof the transmission power amplifier 21, in association with power p(=|x(t)|²) of the transmit signal x(t); the feedback loop 25 for feedingback the output signal y(t) of the transmission power amplifier 21; theAD converter 26 for converting the feedback signal to digital data; thedistortion compensation coefficient calculation unit 27 for calculatingthe distortion compensation coefficient h_(n+1)(p) based upon thetransmit signal x(t) before the distortion compensation thereof and thefeedback signal; the distortion compensation coefficient updating unit28, which has the selector 28 a, for storing the calculated distortioncompensation coefficient h_(n+1)(p) or the corrected distortioncompensation coefficient h_(n+1)(p)′ in association with the power|x(t)|² of the transmit signal x(t); and the comparison unit 29 comparesthe square |h_(n+1)(p)|² of the distortion compensation coefficient thathas been calculated by the distortion compensation coefficient memory 22and the square |h(p)_(MAX)|² of the maximum distortion compensationcoefficient. The apparatus further includes the distortion compensationcoefficient correction unit 30 for correcting the distortioncompensation coefficient h_(n+1)(p) to h_(n+1)(p)/m and outputting thecorrected value h_(n+1)(p)′[=h_(n+1)(p)/m]; the transmit-signal powercalculation unit 31 for generating read/write addresses of thedistortion compensation coefficient memory 22; and delay circuits 32 to39 for adjusting timing.

The apparatus further includes an error signal generator 61 having amultiplier 61 a for reading the distortion compensation coefficienth_(n)(p) that conforms to the power |x(t)|² of the transmit signal outof the memory 22 and subjecting the transmit signal x(t) to complexmultiplication by the distortion compensation coefficient h_(n)(p), anda subtractor 61 b for outputting an error signal E(t), which is thedifference between the multiplier output signal x(t)*h_(n)(p) and thetransmit signal x(t). The apparatus further includes a DA converter 62for converting the digital error signal E(t) to an analog signal; a DAconverter 63 for converting the transmit signal (main signal) x(t) to ananalog signal; and a combiner 64 for combining and outputting the analogtransmit signal x(t) and the analog error signal E(t).

The DA converter 62 subjects only the distortion signal (error signal)E(t) to a digital-to-analog conversion. The upper-limit value of thedistortion compensation coefficient does not depend upon the transmitsignal x(t) and is fixed at a certain value h(p)_(MAX). Accordingly, ifthe distortion compensation coefficient h_(n)(p) conforming to the powerof the transmit signal falls within a circuit of radius h(p)_(MAX)tangent to a DA converter limit LH_(S) (see FIG. 14), the DA converterlimit LH_(S) will not be exceeded, regardless of the phase of thedistortion compensation coefficient h_(n)(p), and neither amplitude norphase will be distorted in the DA converter 24. However, when the outputamplitude of the transmission power amplifier 21 rises owing to theaction of the distortion function f(p), the difference between thetransmit signal x(t) before the distortion compensation thereof and thefeedback signal increases and the distortion compensation coefficienth_(n+1)(p) output from the distortion compensation coefficientcalculation unit 27 grows larger and exceeds the maximum distortioncompensation coefficient h(p)_(MAX). If in such case the distortioncompensation coefficient h_(n+1)(p) is stored in the distortioncompensation coefficient memory 22 as is without being corrected, thisdistortion compensation coefficient will be read out of the memory 22,the distortion-compensated signal h_(n)(p) will exceed the DA converterlimit LH_(S) and both amplitude and phase distortion will be produced inthe DA converter 62.

Accordingly, before the distortion compensation coefficient h_(n+1)(p)is stored in the memory 22 following the calculation of thiscoefficient, the square |h_(n+1)(p)|² of this distortion compensationcoefficient and the square |h(p)_(MAX)|² of the upper-limit distortioncompensation coefficient are compared. When |h_(n+1)(p)|²>|h(p)_(MAX)|²holds, the size of the distortion compensation coefficient is correctedby being multiplied by 1/m while the phase thereof is maintained. Ifthis arrangement is adopted, the DA converter input will be limited inamplitude, the DA converter limit LH_(S) will no longer be exceeded andneither amplitude distortion nor phase distortion will occur in the DAconverter. Moreover, phase follow-up becomes possible because the sizeof the distortion compensation coefficient h_(n+1)(p) is reduced whilethe phase thereof is maintained. It should be noted that the reason forsquaring is that the distortion compensation coefficient h_(n+1)(p) is acomplex number.

Thus, the comparison unit 29 compares the square |h_(n+1)(p)|² of thedistortion compensation coefficient h_(n+1)(p), which has beencalculated by the distortion compensation coefficient calculation unit27, and the square |h(p)_(MAX)|² of the maximum distortion compensationcoefficient. If |h_(n+1)(p)|² is less than |h(p)_(MAX)|², the distortioncompensation coefficient updating unit 28 stores the calculateddistortion compensation coefficient h_(n+1)(p) in the memory 22 withoutcorrecting it. If |h_(n+1)(p)|² is greater than |h(p)_(MAX)|², thedistortion compensation coefficient updating unit 28 stores thedistortion compensation coefficient h_(n+1)(p)′ in the memory 22.

When distortion compensation processing for the next transmit signalx(t) is subsequently executed, the corrected distortion compensationcoefficient h_(n)(p) is read out of the memory 22, the error signal E(t)is output, the combiner 64 combines the analog main signal (the transmitsignal) and the analog error signal, which have been DA-convertedindependently by the DA converters 62 and 63, respectively, and inputsthe resultant signal to the transmission power amplifier. Now thedistortion compensation coefficient h_(n)(p) is smaller than the maximumdistortion compensation coefficient h(p)_(MAX) and, as a result, the DAconverter 62 produces neither amplitude nor phase distortion.

(b) Second Embodiment

FIG. 15 is a diagram illustrating the construction of a secondembodiment of a distortion compensating apparatus for combining ananalog transmit signal and an analog error signal, which have beenDA-converted independently, and inputting the resultant signal to atransmission power amplifier. Components in FIG. 15 identical with thoseof the first embodiment of FIG. 13 are designated by like referencecharacters. This embodiment differs in that the structures of thecomparison unit 29 and distortion compensation coefficient correctionunit 30 are shown in detail.

The comparison unit 29 has a memory 29 g for holding the square|h(p)_(MAX)|² of the maximum distortion compensation coefficient, anarithmetic unit 29 h for calculating the square |h_(n+1)(p)|² of thedistortion compensation coefficient, and a comparator 29 i for comparing|h_(n+1)(p)|² and |h(p)_(MAX)|². The distortion compensation coefficientcorrection unit 30, which corrects the distortion compensationcoefficient h_(n+1)(p) to h_(n+1)(p)/m so that the square |h_(n+1)(p)|²of the distortion compensation coefficient will become smaller than thesquare |h(p)_(MAX)|² of the maximum distortion compensation coefficient,has an m-value controller 30 e and a corrected-value calculation unit 30f. Here m represents a coefficient attenuation ratio and the followingrelation holds:m ² =|h _(n+1)(p)|² /|h(p)_(MAX)|²and m is found fromm=|h _(n+1)(p)|/|h(p)_(MAX)|  (8)The m-value controller 30 e performs the calculation of Equation (8) tocalculate the coefficient attenuation ratio m, and the corrected-valuecalculation unit 30 f outputs the corrected value X of the distortioncompensation coefficient upon calculating the same in accordance withthe following equation:X=h _(n+1)(p)/m

When the distortion compensation coefficient h_(n+1)(p) has been found,the comparison unit 29 compares |h_(n+1)(p)|² and |h(p)_(MAX)|².Further, the distortion compensation coefficient correction unit 30corrects the distortion compensation coefficient h_(n+1)(p) toh_(n+1)(p)/m in such a manner that the square |h_(n+1)(p)|² of thedistortion compensation coefficient will become smaller than the square|h(p)_(MAX)|² of the maximum distortion compensation coefficient. If|h_(n+1)(p)|² is less than |h(p)_(MAX)|², the distortion compensationcoefficient updating unit 28 stores the calculated distortioncompensation coefficient h_(n+1)(p) in the memory 22 as is withoutcorrecting it. If |h_(n+1)(p)|² is greater than |h(p)_(MAX)|², thedistortion compensation coefficient updating unit 28 stores thecorrected value h_(n+1)(p)′ of the distortion compensation coefficientin the memory 22.

When distortion compensation processing for the next transmit signalx(t) is subsequently executed, the corrected distortion compensationcoefficient h_(n)(p) is read out of the memory 22, the error signal E(t)is output, the combiner 64 combines the transmit signal and the errorsignal, which have been DA-converted independently by the DA converters62 and 63, respectively, and inputs the resultant signal to thetransmission power amplifier. Now the distortion compensationcoefficient h_(n)(p) is smaller than the maximum distortion compensationcoefficient h(p)_(MAX) and, as a result, the DA converter 62 producesneither amplitude nor phase distortion.

(c) Third Embodiment

FIG. 16 is a diagram illustrating the construction of a third embodimentof a distortion compensating apparatus for combining an analog transmitsignal and an analog error signal, which have been DA-convertedindependently, and inputting the resultant signal to a transmissionpower amplifier. Components in FIG. 16 identical with those of the firstembodiment of FIG. 13 are designated by like reference characters. Thisembodiment differs in the following respects:

(1) a corrected value h_(n+1)(p)′ of the distortion compensationcoefficient is stored in the distortion compensation value limiter table41 beforehand in association with h_(n+1)(p);

(2) the corrected value h_(n+1)(p)′ of a desired distortion compensationcoefficient is read out of the table 41 and stored in the distortioncompensation coefficient memory 22;

(3) the comparison unit 29 and distortion compensation coefficientcorrection unit 30 are deleted; and

(4) the delay circuits are deleted.

The distortion compensation value limiter table 41 is created asfollows: The distortion compensation coefficient h_(n+1)(p) is correctedto h_(n+1)(p)/m so that the square |h_(n+1)(p)|² of thedistortion-compensated signal will become less than the square|h(p)_(MAX)|² of the maximum distortion compensation coefficient. Thecorrected value h_(n+1)(p)′[=h_(n+1)(p)/m] is then put into table formin association with h_(n+1)(p). In this case, if |h_(n+1)(p)|² issmaller than |h(p)_(MAX)|², the distortion compensation coefficienth_(n+1)(p) is not corrected and is put into table form in associationwith h_(n+1)(p) as is.

When the distortion compensation coefficient calculation unit 27calculates the distortion compensation coefficient h_(n+1)(p) indistortion compensation processing, the distortion compensationcoefficient updating unit 28 obtains, from table 41, the corrected valueh_(n+1)(p)′ of the distortion compensation coefficient conforming to thedistortion compensation coefficient h_(n+1)(p) and stores this value inthe distortion compensation coefficient memory 22.

When distortion compensation processing for the next transmit signalx(t) is subsequently executed, the corrected distortion compensationcoefficient h_(n)(p) is read out of the memory 22, the error signal E(t)is output, the combiner 64 combines the transmit signal and the errorsignal, which have been DA-converted independently by the DA converters62 and 63, respectively, and inputs the resultant signal to thetransmission power amplifier. Now the distortion compensationcoefficient h_(n)(p) is smaller than the maximum distortion compensationcoefficient h(p)_(MAX) and, as a result, the DA converter 62 producesneither amplitude nor phase distortion.

(d) Fourth Embodiment

FIG. 17 is a diagram illustrating the construction of a fourthembodiment of a distortion compensating apparatus for combining ananalog transmit signal and an analog error signal, which have beenDA-converted independently, and inputting the resultant signal to atransmission power amplifier. Components in FIG. 17 identical with thoseof FIG. 13 are designated by like reference characters. This embodimentdiffers in the location of the table 41. In the third embodiment, thetable 41 is provided on the input side of the distortion compensationcoefficient memory 22. In the fourth embodiment, however, the table 41is provided on the output side of the distortion compensationcoefficient memory 22. In the fourth embodiment, in other words, thedistortion compensation coefficient is corrected using the table 41after the distortion compensation coefficient is read out of the memory22.

When the distortion compensation coefficient calculation unit 27calculates the distortion compensation coefficient h_(n+1)(p) indistortion compensation processing, the distortion compensationcoefficient updating unit 28 stores the distortion compensationcoefficient h_(n+1)(p) in the distortion compensation coefficient memory22 as is. When distortion compensation processing for the next transmitsignal x(t) is executed and the distortion compensation coefficienth_(n)(p) is read out of the memory 22, the error signal generator 61obtains the distortion compensation coefficient h_(n)(p)′ conforming tothe distortion compensation coefficient h_(n)(p) from the table 41 andoutputs the error signal E(t), and the combiner 64 combines the transmitsignal and the error signal, which have been DA-converted independentlyby the DA converters 62 and 63, respectively, and inputs the resultantsignal to the transmission power amplifier 21.

(e) Embodiment Applied to Multicarrier Transmitter

The first to fourth embodiments are examples in which the invention isapplied to a single-carrier transmitter. However, the invention can beapplied to a multicarrier transmitter as well. FIG. 18 is a blockdiagram showing the construction of a transmitter in a case where aplurality of transmit signals are transmitted using a multicarriersignal. This illustrates an example of a case where four frequencies aremultiplexed and transmitted.

Transmit signals x₁(t), x₂(t), x₃(t), x₄(t) of the respective carriersare converted to analog signals by independent DA converters 71 ₁, 71 ₂,71 ₃, 71 ₄, respectively. Upon passing through filters 72 ₁, 72 ₂, 72 ₃,72 ₄, the analog signals are frequency-converted to desired carrierfrequencies f₁, f₂, f₃, f₄ [see (a) in FIG. 19] by frequency converters73 ₁, 73 ₂, 73 ₃, 73 ₄, respectively, and these frequencies are thenfrequency-multiplexed by a combiner 74.

The frequency-multiplexed signal (main signal) SM obtained is combinedwith the error signal E(t), which is output from the error signalgenerator 61, by the combiner 64, and the resultant signal enters thetransmission power amplifier 21. Part of the output of the transmissionpower amplifier 21 is frequency-converted to a multiplexed signal offrequencies f₁-f₀, f₂-f₀, f₃-f₀, f₄-f₀ by a frequency converter 75, thesignal passes through a filter 76 and is AD-converted by the ADconverter 26, and the digital signal is input to the distortioncompensation coefficient calculation unit 27 as a feedback signal S_(F).

Meanwhile, the transmit signals x₁(t), x₂(t), x₃(t), x₄(t) aremultiplied by expj(ω₁-ω₀)t, expj(ω₂-ω₀)t, expj(ω₃-ω₀)t, expj(ω₄-ω₀)t(where ω_(n)=2πf_(n)), respectively, by frequency shifters 77 ₁, 77 ₂,77 ₃, 77 ₄, respectively, to effect a frequency shift to the frequenciesf₁-f₀, f₂-f₀, f₃-f₀, f₄-f₀ [see (b) in FIG. 19], after which thesefrequencies are frequency-multiplexed by a combiner 78. The multiplexedsignal is input to the distortion compensating apparatus as areferential signal S_(R).

The distortion compensating apparatus outputs an error signal E, whichgives rise to non-linear distortion in the transmission power amplifier21, upon calculating the error signal using the referential signal S_(R)and the feedback signal S_(F). The DA converter 62 DA-converts the errorsignal E obtained and inputs the analog signal to a frequency converter80 via a filter 79. The frequency converter 80 multiplies the errorsignal E by a signal of frequency f₀ to thereby effect an up-conversionto an error signal of frequencies f₁, f₂, f₃, f₄. The combiner 64combines the main signal (transmit signal) S_(M) of frequencies f₁, f₂,f₃, f₄ and the error signal E of frequencies f₁, f₂, f₃, f₄ and inputsthe combined signal to the transmission power amplifier 21. Thus thereis obtained a signal that is the result of providing thefrequency-multiplexed signal (main signal) with a characteristic that isthe reverse of the non-linear distortion of the amplifier.

FIG. 20 illustrates a fifth embodiment of a case where the distortioncompensating apparatus according to the first embodiment of FIG. 13 isapplied to the multicarrier transmitter of FIG. 18. Components identicalwith those in FIGS. 13 and 18 are designated by like referencecharacters. Here numerals 70 ₁ to 70 ₃ denote delay circuits foradjusting timing.

FIG. 21 illustrates a sixth embodiment of a case where the distortioncompensating apparatus according to the second embodiment of FIG. 15 isapplied to the multicarrier transmitter of FIG. 18. Components identicalwith those in FIGS. 15 and 18 are designated by like referencecharacters.

FIG. 22 illustrates a seventh embodiment of a case where the distortioncompensating apparatus according to the third embodiment of FIG. 16 isapplied to the multicarrier transmitter of FIG. 18. Components identicalwith those in FIGS. 16 and 18 are designated by like referencecharacters.

FIG. 23 illustrates an eighth embodiment of a case where the distortioncompensating apparatus according to the fourth embodiment of FIG. 17 isapplied to the multicarrier transmitter of FIG. 18. Components identicalwith those in FIGS. 17 and 18 are designated by like referencecharacters.

(f) Embodiment Applied to Different Multicarrier Transmitter

FIG. 24 is a block diagram showing the construction of anothertransmitter in a case where a plurality of transmit signals aretransmitted using a multicarrier signal. This illustrates an example ofa case where four frequencies are multiplexed and transmitted.

Transmit signals x₁(t), x₂(t), x₃(t), x₄(t) of the respective carriersare multiplied by expjω₁t, expjω₂t, expjω₃t, expjω₄t (whereω_(n)=2πf_(n)), respectively, by frequency shifters 91 ₁, 91 ₂, 91 ₃, 91₄, respectively, to effect a frequency shift to the frequencies f₁, f₂,f₃, f₄ [see (a) in FIG. 25], after which these frequencies are convertedto analog signals by independent DA converters 92 ₁, 92 ₂, 92 ₃, 92 ₄.The analog signals are frequency-multiplexed by a combiner 93. Thefrequency-multiplexed signal passes through a low-pass filter 99 andthen is shifted to high-frequency bands f₀-f₁, f₀-f₂, f₀-f₃, f₀-f₄ by afrequency shifter 100 (see (b) in FIG. 25) to obtain a main signalS_(M), which enters the combiner 64. Thereafter, thefrequency-multiplexed signal (main signal) S_(M) is combined with theerror signal E, which is output from the error signal generator 61, bythe combiner 64, and the resultant signal is input to the transmissionpower amplifier 21. Part of the output of the transmission poweramplifier 21 is frequency-converted to a low-frequency band multiplexedsignal of frequencies f₁, f₂, f₃, f₄ by a frequency converter 94, thesignal passes through a filter 95 and is AD-converted by the ADconverter 26, and the digital signal is input to the distortioncompensation coefficient calculation unit 27 as the feedback signalS_(F).

The outputs of the frequency shifters 91 ₁-91 ₄ arefrequency-multiplexed by a combiner 96, after which the resultant signalis input to the distortion compensating apparatus as the referentialsignal S_(R).

The distortion compensating apparatus outputs an error signal E, whichgives rise to non-linear distortion in the transmission power amplifier21, upon calculating the error signal using the referential signal S_(R)and the feedback signal S_(F). The DA converter 62 DA-converts the errorsignal E obtained and inputs the analog signal to a frequency converter98 via a filter 97. The frequency converter 98 multiplies the errorsignal E of frequencies f₁, f₂, f₃, f₄ by a signal of frequency f₀ tothereby effect a shift to high-frequency bands f₀-f₁, f₀-f₂, f₀-f₃,f₀-f₄. The combiner 64 combines the main signal (transmit signal) S_(M)and the error signal E and inputs the combined signal to thetransmission power amplifier 21. Thus there is obtained a signal that isthe result of providing the frequency-multiplexed signal (main signal)with a characteristic that is the reverse of the non-linear distortionof the amplifier.

FIG. 26 illustrates a ninth embodiment of a case where the distortioncompensating apparatus according to the first embodiment of FIG. 13 isapplied to the multicarrier transmitter of FIG. 24. Components identicalwith those in FIGS. 13 and 24 are designated by like referencecharacters. Reference characters 90 ₁, to 90 ₃ denote delay circuits foradjusting timing.

FIG. 27 illustrates a tenth embodiment of a case where the distortioncompensating apparatus according to the second embodiment of FIG. 15 isapplied to the multicarrier transmitter of FIG. 24. Components identicalwith those in FIGS. 16 and 24 are designated by like referencecharacters.

FIG. 28 illustrates an 11th embodiment of a case where the distortioncompensating apparatus according to the third embodiment of FIG. 16 isapplied to the multicarrier transmitter of FIG. 24. Components identicalwith those in FIGS. 17 and 24 are designated by like referencecharacters.

FIG. 29 illustrates a 12th embodiment of a case where the distortioncompensating apparatus according to the fourth embodiment of FIG. 17 isapplied to the multicarrier transmitter of FIG. 24. Components identicalwith those in FIGS. 15 and 24 are designated by like referencecharacters.

(D) Distortion Compensating Apparatus Having Function for ControllingAmplitude of Feedback Signal

In the foregoing embodiments, a distortion compensation coefficient iscorrected to exercise control in such a manner that a transmit signalthat has undergone distortion compensation processing will not exceed alimit level. In the embodiments that follow, however, the amplitude of afeedback signal from a transmission power amplifier is controlled sothat a transmit signal that has undergone distortion compensationprocessing will not exceed a limit level.

(a) First Embodiment

FIG. 30 is a diagram showing the construction of a distortioncompensating apparatus according to a first embodiment for controllingthe amplitude of the feedback signal y(t) based upon the amplitude orpower of the transmit signal x(t). The apparatus includes the device(the transmission power amplifier) 21 which produces non-lineardistortion of a function f(p); the distortion compensation coefficientmemory 22 for storing the distortion compensation coefficient h(p),which corrects the distortion of the transmission power amplifier 21, inassociation with power p (=|x(t)|²) of the transmit signal x(t); apredistortion unit 23 for reading the distortion compensationcoefficient h_(n)(p) conforming to the power p of the transmit signalx(t) out of the memory 22 and applying distortion compensationprocessing[=x(t)*h_(n)(p)] to the transmit signal x(t) using thisdistortion compensation coefficient h_(n)(p); the DA converter 24 forconverting the digital transmit signal x(t)*h_(n)p, which is the resultof the applied distortion compensation processing, to an analog signal;the feedback loop 25 for feeding back the output signal y(t) of thetransmission power amplifier 21; the AD converter 26 for converting theoutput signal, namely the feedback signal y(t), to digital data; thedistortion compensation coefficient calculation unit 27 for calculatinga distortion compensation coefficient h_(n+1)(p) based upon the transmitsignal x(t) before the distortion compensation thereof and the feedbacksignal y(t); the transmit-signal power calculation unit 31 forgenerating read/write addresses of the distortion compensationcoefficient memory 22; and an amplitude controller 81 for controllingthe amplitude of the feedback signal y(t) based upon the amplitude orpower |x(t)|² of the transmit signal x(t) before the latter iscompensated for distortion.

The distortion compensation coefficient calculation unit 27 has astructure identical with that shown in FIG. 3 but only the subtractor 27b for calculating the difference e(t) between the transmit signal x(t)prior to the distortion compensation thereof and the feedback signal isshown. The other components are illustrated as a main unit 27′ forcalculating the distortion compensation coefficient. The amplitudecontroller 81 has a gain setting unit 81 a for storing the correspondingrelationship between the amplitude or power of the transmit signal x(t)and gain and for outputting a gain G that conforms to the transmitsignal x(t), and a multiplier 81 b for multiplying the feedback signaly(t) by the gain G.

The amplitude and phase of the output signal are not distorted in thelinear region of the transmission power amplifier 21. In the non-linearregion of the transmission power amplifier 21, however, the differencee(t) between the transmit signal x(t) before the distortion compensationthereof and the feedback signal y(t) increases and so does thedistortion compensation coefficient h_(n+1)(p) output from thedistortion compensation coefficient calculation unit 27. The distortioncompensation coefficient h_(n+1)(p) eventually is read out and thepredistortion unit 23 outputs the distortion-compensated signalx(t)*h_(n+1)(p). If the distortion-compensated signal x(t)*h_(n+1)(p)exceeds the DA converter limit, amplitude distortion and phasedistortion occur in the DA converter. The larger the transmit signal,the greater the tendency for the above. If some measures are not taken,therefore, amplitude and phase distortion will occur in the non-linearregion of the power amplifier.

Accordingly, in the non-linear region, the amplitude controller 81performs control based upon the uncompensated transmit signal x(t) so asto raise the amplitude of the feedback signal y(t) in such a manner thatthe difference e(t) between the two signals will not increase. If thisarrangement is adopted, the distortion compensation coefficienth_(n+1)(p) can be prevented from increasing, the distortion-compensatedsignal x(t)*h_(n+1)(p) can be prevented from exceeding the DA converterlimit and amplitude and phase distortion can be prevented fromoccurring.

It should be noted that the difference e(t) between the transmit signalx(t) and feedback signal y(t) increases as the degree of non-linearityof the transmission power amplifier rises, i.e., as the level of theuncompensated transmit signal x(t) rises. Accordingly, a gain vs.amplitude characteristic (or gain vs. power characteristic) illustratedin any one of FIGS. 31A to 31C is set in the gain setting unit 81 a.FIG. 31A illustrates a characteristic in which the gain G is fixed at 1in the linear region of the transmission power amplifier 21 and rises inaccordance with a linear function of the transmit-signal amplitude (orpower) in the non-linear region. FIG. 31B illustrates a characteristicin which the gain G rises in accordance with a quadratic function of thetransmit-signal amplitude (or power) in the non-linear region. FIG. 31Cillustrates a characteristic in which the gain G rises in steps inaccordance with the transmit-signal amplitude (or power) in thenon-linear region.

Since gain G=1 holds when the transmit signal x(t) prior to thedistortion compensation thereof is below a linear/non-linear boundarysignal X_(B), the amplitude controller 81 does not change the level ofthe feedback signal y(t). However, if the transmit signal x(t) prior tothe distortion compensation thereof exceeds the linear/non-linearboundary signal X_(B), the gain G becomes greater than 1 in accordancewith the set function of the gain setting unit 81 a. As a consequence,the amplitude controller 81 outputs y(t)′=G·y(t) (G>1) to reduce thedifference e(t) output from the subtractor 27 b. As a result, thedistortion compensation coefficient h_(n+1)(p) can be prevented fromincreasing, the distortion-compensated signal x(t)*h_(n+1)(p) can beprevented from exceeding the DA converter limit and the occurrence ofamplitude and phase distortion can be suppressed.

The foregoing relates to a case where gain is controlled in accordancewith the characteristics of FIGS. 31A to 31C in the non-linear region.However, gain can be controlled in accordance with any function withoutbeing limited to these characteristics. Further, in the case describedabove, gain G is varied instantaneously. However, an arrangement may beadopted in which gain is varied up to the set value asymptotically intime in the manner of an exponential function, or in which gain isvaried up to the set value in the manner of a linear function. In otherwords, the function of the gain G is chosen taking into considerationthe characteristics of the power amplifier and the characteristics ofthe feedback loop. Time control of the gain G also is decided in similarfashion.

FIGS. 32 and 33 illustrate first and second modifications of the firstembodiment of the present invention. These are examples in which theamplitude of the feedback signal y(t) is controlled in analog fashionbefore an analog-to-digital conversion is performed. In FIG. 32, theamplitude controller 81 is provided on the input side of the ADconverter 26, controls the gain G of a variable-gain amplifier (VGA) 81c in accordance with the level of the transmit signal x(t), amplifiesthe feedback signal y(t) by the variable-gain amplifier (VGA) 81 c andoutputs the resultant signal. In FIG. 33, the amplitude controller 81 isprovided on the input side of the AD converter 26, controls the amountof attenuation of a variable attenuator (VATT) 81 d in accordance withthe level of the transmit signal x(t), attenuates the feedback signaly(t) a predetermined amount by the variable attenuator 81 d, thenamplifies the output of the attenuator by a constant-gain amplifier 81 eand outputs the resultant signal.

The modifications of FIGS. 32 and 33 can be applied to the embodimentsbelow as well.

(b) Second Embodiment

FIG. 34 is a diagram showing the construction of a distortioncompensating apparatus according to a second embodiment for controllingthe amplitude of the feedback signal y(t). Components identical withthose of the first embodiment of FIG. 30 are designated by likereference characters. This second embodiment differs from the firstembodiment in that (1) a DAC-limit surpass detector 82 (for example, asillustrated in FIG. 34) is provided in the second embodiment fordetecting whether the transmit signal after the distortion compensationthereof has surpassed a DA converter limit LML (see FIG. 2); (2) theamplitude controller 81 controls the amplitude of the feedback signaly(t) when the transmit signal after the distortion compensation thereofhas surpassed the DA converter limit LML; and (3) a fixed gain G0 (>1)that is independent of the level of the transmit signal x(t) has beenset in the gain setting unit 81 a.

If the transmit signal (distortion-compensated signal) x′(t) after thedistortion compensation thereof exceeds the DA converter limit LM_(L),the amplitude and the phase of the feedback signal y(t) become distortedand the amplitude declines. Accordingly, the DAC-limit surpass detector82 determines whether the transmit signal after the distortioncompensation thereof has surpassed the DA converter limit LM_(L). Ifsuch is the case, then the amplitude controller 81 multiplies thefeedback signal y(t) by the fixed gain G₀(>1). As a result, thedifference e(t) output from the subtractor 27 b decreases, thedistortion compensation coefficient h_(n+1)(p) can be prevented fromincreasing, the distortion-compensated signal can be prevented fromexceeding the DA converter limit LML and the occurrence of amplitude andphase distortion can be suppressed thereafter.

(c) Third Embodiment

FIG. 35 is a diagram showing the construction of a distortioncompensating apparatus according to a third embodiment for controllingthe amplitude of the feedback signal y(t) based upon the amplitude orpower of the transmit signal x(t). Components identical with those ofthe second embodiment are designated by like reference characters. Thisembodiment differs in that (1) the gain G is not fixed; (2) the gainsetting unit 81 a is provided with a gain table in which thecharacteristic of any one of FIGS. 31A to 31C has been sent; and (3)when the distortion-compensated signal x′(t) has exceeded the DAconverter limit LM_(L), the amplitude controller 81 controls the gain Gbased upon the level of the transmit signal x(t).

The DAC-limit surpass detector 82 determines whether thedistortion-compensated signal x′(t) output from the predistortion unit23 has surpassed the DA converter limit LM_(L). If thedistortion-compensated signal x′(t) is a value within the DA converterlimit, the gain setting unit 81 a of the amplitude controller 81 outputsa gain G of 1 and changes the amplitude of the feedback signal.

If the distortion-compensated signal x′(t) surpasses the DA converterlimit, however, the DAC-limit surpass detector 82 instructs theamplitude controller 81 to change over the gain. In response, the gainsetting unit 81 a reads a gain G (>1) conforming to the level of thetransmit signal x(t) out of a gain table (not shown) and inputs the gainto the multiplier 81 b. The latter multiplies the feedback signal y(t)by the gain G (>1) and outputs the signal y(t)′[=G·y(t)]. As a result,the difference e(t) output from the subtractor 27 b decreases, thedistortion compensation coefficient h_(n+1)(p) does not increase, thedistortion-compensated signal no longer exceeds the DA converter limitand amplitude and phase distortion no longer occur.

In accordance with the third embodiment, gain is controlled based uponthe level of the transmit signal x(t). As a result, control forpreventing the occurrence of amplitude and phase distortion can beperformed more finely in comparison with the second embodiment in whichgain is fixed.

(d) Fourth Embodiment

FIG. 36 is a diagram showing the construction of a distortioncompensating apparatus according to a fourth embodiment for controllingthe amplitude of the feedback signal y(t) based upon the amplitude orpower of the transmit signal x(t). Components identical with those ofthe third embodiment are designated by like reference characters. Thisembodiment differs in that (1) a multiplier 83 is provided formultiplying the power |x(t)|² of the uncompensated transmit signal x(t)by a factor k (where k is a constant); (2) an arithmetic unit 84 isprovided for calculating the power |x′(t)|² of the transmit signalx′(t); (3) an arithmetic unit 85 is provided for calculating thedifference between k·|x(t)|², which is k times the transmission signalpower, and the power |x′(t)|² of the distortion-compensated signal; (4)a difference-signal processing unit 86 is provided for instructing theamplitude controller 81 to start control of the amplitude of thefeedback signal y(t) when the power |x′(t)|² of thedistortion-compensated signal is greater than k·|x(t)|², which is ktimes the transmission signal power; and (5) the amplitude controller 81controls the amplitude of the feedback signal y(t) in response to thecommand to start amplitude control. It should be noted that k is anaverage value of distortion compensation coefficients that have beenstored in the distortion compensation coefficient memory 22 or a fixedvalue that conforms to the type of transmission power amplifier 21.

FIG. 37 illustrates the overall processing flow of the distortioncompensating apparatus according to the fourth embodiment. The DAC-limitsurpass detector 82 checks to determine whether thedistortion-compensated signal x′(t) output from the predistortion unit23 has surpassed the DA converter limit LM_(L) (step 101). If thedistortion-compensated signal x′(t) falls within the DA converter limit,the gain setting unit 81 a outputs gain G (=1) and therefore does notchange the amplitude of the feedback signal.

If the distortion-compensated signal x′(t) surpasses the DA converterlimit, however, the DAC-limit surpass detector 82 instructs thearithmetic unit 84 to calculate the power |x′(t)|² of thedistortion-compensated signal. The arithmetic unit 84 responds bycalculating the power |x′(t)|² of the distortion-compensated signal(step 102). Further, the power calculation unit 31 calculates the power|x(t)|² of the transmit signal and the multiplier 83 performs thecalculation k·|x(t)|² (steps 103, 104). Next, the arithmetic unit 85performs the calculation indicated by the following equation:d=|x′(t)|² −k·|x(t)|²  (9)and inputs the result d to the difference-signal processing unit 86. Thelatter determines whetherd=|x′(t)|² −k·|x(t)|²>0  (10)holds (step 105). If the decision rendered is “YES”, then thedifference-signal processing unit 86 instructs the amplitude controller81 to update the gain. In response, the gain setting unit 81 a of theamplitude controller 81 reads the gain G (>1), which conforms to thelevel of the transmit signal x(t), out of the gain table and inputs thegain to the multiplier 81 b (step 106).

The multiplier 81 b subsequently multiplies the feedback signal y(t) bythe gain G (>1) and outputs the signal y(t)′[=G·y(t)]. As a result, thedifference e(t) output from the subtractor 27 b decreases, thedistortion compensation coefficient h_(n+1)(p) does not increase, thedistortion-compensated signal no longer exceeds the DA converter limitand amplitude and phase distortion no longer occur.

In the description rendered above, the power difference between thetransmit signal and the distortion-compensated signal is calculated bythe arithmetic unit 85 and gain is changed over based upon the powerdifference to control the amplitude of the feedback signal. However, theamplitude of the feedback signal can be controlled also by changing overthe gain based upon an amplitude difference between the transmit signaland the distortion-compensated signal.

In accordance with the fourth embodiment, the gain G is controlled basedupon the amplitude or power of the transmit signal x(t) only when thedistortion-compensated signal x′(t) exceeds the DAC limit and thedifference between the transmit signal x(t) and thedistortion-compensated signal x′(t) increases. As a consequence, gain iscontrolled upon making sure that such control is truly necessary, andgain is not controlled when control is unnecessary. This makes possiblefiner control to suppress amplitude and phase distortion.

FIG. 38 is a first modification of the fourth embodiment, in whichcomponents identical with those of the fourth embodiment are designatedby like reference characters. This modification differs from the fourthembodiment in that (1) when Equation (10) holds (i.e., d>0), thedifference-signal processing unit 86 instructs the amplitude controller81 to start control of the amplitude of the feedback signal, andinstructs the distortion compensation coefficient calculation unit 27 tohalt the calculation of the distortion compensation coefficient when thedifference d exceeds a threshold value D_(TH); and (2) in response tothe command to halt calculation, the distortion compensation coefficientcalculation unit 27 halts the calculation/updating of the distortioncompensation coefficient. If d exceeds the threshold value D_(TH), thismeans that the distortion compensating effect of the calculateddistortion compensation coefficient is doubtful, i.e., that thedistortion compensation coefficient has become less reliable.Accordingly, if the difference d has exceeded the threshold valueD_(TH), then, until the relation d<D_(TH) is established, updating ofthe distortion compensation coefficient is not carried out and thedistortion-compensated signal is thenceforth generated in accordancewith the distortion compensation coefficient obtained thus far.

If d in Equation (9), namely the difference between k times the power|x(t)|² of the transmit signal and the power |x′(t)|² of thedistortion-compensated signal, is equal to or less than zero (d≦0),then, even if the distortion-compensated signal x′(t) exceeds the DAClimit, the gain setting unit 81 a inputs G=1 to the multiplier 81 b anddoes not change the amplitude of the feedback signal y(t). However, ifthe distortion-compensated signal x′(t) exceeds the DAC limit and,moreover, d>0 holds, then the gain setting unit 81 a reads the gain G(>1) conforming to the level of the transmit signal x(t) out of a gaintable and inputs the gain to the multiplier 81 b. The latter multipliesthe feedback signal y(t) by the gain G (>1) and outputs the signaly(t)′[=G·y(t)]. As a result, the difference e(t) output from thesubtractor 27 b decreases, the distortion compensation coefficienth_(n+1)(p) does not increase, the distortion-compensated signal nolonger exceeds the DA converter limit and amplitude and phase distortionno longer occur.

Further, when the difference d increases further and the threshold valueD_(TH) is exceeded, the difference-signal processing unit 86 instructsthe distortion compensation coefficient calculation unit 27 to halt thecalculation of the distortion compensation coefficient. In response, thedistortion compensation coefficient calculation unit 27 stopscalculating the distortion compensation coefficient, as a result ofwhich distortion compensation coefficient is not updated. Thus, when thedifference d between k times the power of the transmit signal and thepower of the distortion-compensated signal exceeds the threshold valueD_(TH), updating of the distortion compensation coefficient is halted.This makes it possible to prevent a distortion compensation coefficientfrom taking on a value of doubtful effectiveness.

FIG. 39 is a second modification of the fourth embodiment, in whichcomponents identical with those of the fourth embodiment are designatedby like reference characters. This modification differs from the fourthembodiment in that (1) a μ generator 27 g is provided for generating astep-size parameter μ used in calculation of distortion compensationcoefficients; (2) when Equation (10) holds (i.e., d>0), thedifference-signal processing unit 86 instructs the amplitude controller81 to start control of the amplitude of the feedback signal and inputsthe difference d to the μ generator 27 g; and (3) the μ generator 27 gcontrols the step-size parameter μ based upon the value of thedifference d.

As shown in FIG. 40, convergence time required for compensation ofdistortion to end so as to satisfy the necessary ACPR is dependent uponthe magnitude of the step-size parameter μ; the larger the parameter μ,the shorter the convergence time. If μ is too large, however, stabilityin the vicinity of the target value declines. Accordingly, the value ofμ is controlled based upon the size of the difference d and theconvergence time is shortened while taking the stability of linearizerconvergence into consideration. For example, if a situation arises inwhich the difference d exceeds a threshold value and divergence beginsto develop in the distortion-compensation control loop, the value of μis increased to prevent such divergence immediately. At this time thegain G may be made a constant or may be controlled dynamically (e.g., inaccordance with a linear function) so as to accommodate the value of μ.Further, if the difference d decreases, μ is reduced accordingly toeffect a return to a steady value. If this arrangement is adopted, thedifference e(t) output from the subtractor 27 b can be reduced in ashort period of time and control can be carried out in such a mannerthat the amplitude of the distortion-compensated signal will not exceeda limit value.

(e) Fifth Embodiment

The first to fourth embodiments are examples in which the invention isapplied to a single-carrier transmitter. However, the invention can beapplied to a multicarrier transmitter as well. FIG. 41 is a diagramshowing the construction of a distortion compensating apparatus(multicarrier linearizer) for a case where a plurality of transmitsignals are transmitted using a multicarrier signal. This illustrates anexample of a case where four frequencies are multiplexed andtransmitted. The digital transmit signals x₁(t), x₂(t), x₃(t), x₄(t) aremultiplied by exp(jω₁t), exp(jω₂t), exp(jω₃t), exp(jω₄t)(ω_(n)=2πf_(n)), respectively, by the frequency shifters 51, 52, 53, 54,respectively, to effect a frequency shift to frequencies f₁, f₂, f₃, f₄,after which these frequencies are frequency-multiplexed by the combiner55. The digital frequency-multiplexed signal corresponds to the transmitsignal x(t) of the single-carrier distortion compensating apparatus (seeFIG. 30) and subsequently undergoes distortion compensation processingsimilar to the processing executed in the case of the single carrier.FIG. 42 is a diagram useful in describing the effects of themulticarrier linearizer, in which the solid line indicates a spectrumcharacteristic without distortion compensation and the dashed line aspectrum characteristic with distortion compensation.

FIG. 41 is an example in which the distortion compensating apparatus ofthe first embodiment has been adapted so as to be capable ofmulticarrier transmission. However, the distortion compensatingapparatus of the second to fourth embodiments also can be adapted formulticarrier transmission in a similar manner.

(f) Sixth Embodiment

The first to fifth embodiments are examples in which the invention isapplied to a distortion compensating apparatus in which the transmitsignal x(t) is multiplied by the distortion compensation coefficienth_(n)(p) to generate the distortion-compensated signal x′(t) and thelatter is input to the transmission power amplifier 21. However, theinvention is applicable also to a distortion compensating apparatus inwhich a main signal (transmit signal) x(t) and a distortion component(error signal) E(t) appended to the transmit signal are DA-convertedindependently of each other and then combined and input to thetransmission power amplifier.

FIG. 43 illustrates a sixth embodiment of a distortion compensatingapparatus for combining the analog transmit signal and the error signalE(t). Components in FIG. 43 identical with those of the embodiments thusfar are designated by like reference characters.

As shown in FIG. 43, the apparatus includes the device (the transmissionpower amplifier) 21 which produces non-linear distortion of a functionf(p); the distortion compensation coefficient memory 22 for storing thedistortion compensation coefficient h(p), which corrects the distortionof the transmission power amplifier 21, in association with power p(=|x(t)|²) of the transmit signal x(t); the predistortion unit 23 foroutputting a distortion-compensated signal; the feedback loop 25 forfeeding back the output signal y(t) of the transmission power amplifier21; the AD converter 26 for converting the feedback signal y(t) todigital data; the distortion compensation coefficient calculation unit27 for calculating the distortion compensation coefficient h_(n+1)(p)based upon the transmit signal x(t) before the distortion compensationthereof and the feedback signal y(t); the transmit-signal powercalculation unit 31 for generating read/write addresses of thedistortion compensation coefficient memory 22; and the amplitudecontroller 81 for controlling the amplitude of the feedback signal y(t)based upon the amplitude or power of the transmit signal x(t) before thelatter is compensated for distortion. The arrangement set forth above issimilar to that of the first embodiment and the components operate inthe same manner as those of the first embodiment.

The predistortion unit 23 includes the error signal generator 61, whichhas the multiplier 61 a for reading the distortion compensationcoefficient h_(n)(p) that conforms to the power |x(t)|² of the transmitsignal out of the memory 22 and multiplying the transmit signal x(t) bythe distortion compensation coefficient h_(n)(p), and the subtractor 61b for outputting the error signal E(t), which is the difference betweenthe multiplier output signal x(t)*h_(n)(p) and the transmit signal x(t);the DA converter 62 for converting the digital error signal E(t) to ananalog signal; the DA converter 63 for converting the transmit signal(main signal) x(t) to an analog signal; and the combiner 64 forcombining and outputting the analog transmit signal x(t) and the analogerror signal E(t).

When the transmit signal x(t) is subjected to distortion compensationprocessing, the error signal generator 61 reads the distortioncompensation coefficient h_(n)(p) out of the memory 22 and outputs theerror signal E(t), and the combiner 64 combines the analog main signal(the transmit signal) and the analog error signal, which have beenDA-converted independently by the DA converters 62 and 63, respectively,and inputs the resultant signal to the transmission power amplifier.Since the amplitude of the error signal is small, the bit precision ofthe DA converter 62, which outputs only the error signal, can bereduced. Further, since the DA converter 63, which outputs only thetransmit signal, does not require a large dynamic range, the bitprecision of the DA converter 63 can be reduced.

When the transmit signal x(t) is below the linear/non-linear boundarysignal X_(B) (see FIG. 31), the gain G is made 1, the level of thefeedback signal y(t) is not changed and the signal is input to thedistortion compensation coefficient calculation unit 27 as is. Thedistortion compensation coefficient calculation unit 27 calculates thedifference e(t) between the transmit signal x(t) and the feedback signaly(t), calculates the distortion compensation coefficient h_(n+1)(p)based upon the difference e(t) and stores the result in the distortioncompensation coefficient memory 22.

If the transmit signal x(t) exceeds the linear/non-linear boundarysignal X_(B), on the other hand, the amplitude controller 81 controlsthe gain G in accordance with the transmit signal x(t) so that therelation G>1 is established. Consequently, the transmit signal x(t)exceeds the linear/non-linear boundary signal X_(B) and the feedbacksignal y(t) declines. Even though the difference e(t) increases, theamplitude controller 81 immediately outputs y(t)′=G·y(t) (G>1) to reducethe difference e(t) output from the subtractor 27 b. As a result, thedistortion compensation coefficient h_(n+1)(p) can be prevented fromincreasing, the distortion-compensated signal can be prevented fromexceeding the DA converter limit and the occurrence of amplitude andphase distortion can be suppressed.

The sixth embodiment is an example in which the invention is applied toa single-carrier transmitter. However, the invention can be applied to amulticarrier transmitter as well. FIG. 44 is a block diagram showing theconstruction of a transmitter in which a plurality of transmit signalsare transmitted using a multicarrier signal. This is an example in whichthe multicarrier arrangement of FIG. 24 is applied to the sixthembodiment of FIG. 43. Components in FIG. 44 identical with those shownin FIG. 24 are designated by like reference characters.

As shown in FIG. 44, digital transmit signals x₁(t)˜x₄(t) are subjectedto a digital frequency shift decided by the carrier spacing, and a firstfrequency multiplexer 96 multiplexes the frequency-shifted signals andoutputs the frequency-multiplexed signal as a digital transmit signalS_(R). The frequency-shifted signals are converted to analog signals bythe DA converters 92 ₁˜92 ₄, and a second frequency multiplexer 93multiplexes the analog signals and outputs the frequency-multiplexedsignal as an analog transmit signal. The error signal generator 61generates the error signal E based upon the frequency-multiplexeddigital transmit signal S_(R), the DA converter 62 converts the errorsignal to an analog signal, and the combiner 64 combines the output ofthe DA converter 62 and the analog transmit signal S_(M) and inputs theresultant signal to the transmission power amplifier 21.

Though the present invention has been described on the basis of theforegoing embodiments, the present invention can be modified in variousways in accordance with the gist of the invention set forth in theclaims and all such modifications are covered by the invention.

Thus, in accordance with the present invention, the size of a distortioncompensation coefficient is corrected in advance, while the phasethereof is maintained as is, in such a manner that the input amplitudeof a DA converter will not exceed the limit of the DA converter. Thismakes it possible to follow up phase even if amplitude is limited. As aresult, the distortion characteristic will not be degraded beyond thedistortion characteristic obtained when no distortion compensation isapplied.

Further, in accordance with the present invention, the size of adistortion compensation coefficient is corrected in advance, while thephase thereof is maintained as is, in such a manner that the power of atransmit signal after the distortion compensation thereof will notexceed an allowed upper-limit power. This makes it possible to follow upphase even if amplitude is limited.

Further, in accordance with the present invention, processing for acomparison operation and for calculating corrected values can bedispensed with by putting corrected values of a distortion compensationcoefficient into table form, thereby making it possible to simplify thearrangement and to obtain corrected values in simple fashion.

Further, in accordance with the present invention, the calculationnecessary when calculating a corrected value can be simplified by a bitshift through approximation of a denominator by an exponent of 2.

Further, in accordance with the present invention, the correspondencebetween the amplitude (or power) of an uncompensated transmit signal andgain is put into the form of a table, a gain that conforms to an actualtransmit-signal amplitude or power is found from the table and theamplitude of the feedback signal from a transmission power amplifier iscontrolled based upon the gain found. This makes it possible to arrangeit so that the amplitude of the distortion-compensated signal will notexceed a limit value owing feed-forward of a signal. As a result,distortion compensation can be performed in stable fashion, withoutlosing significant components (amplitude and phase) of the signal,through a simple arrangement.

Further, in accordance with the present invention, when the amplitude ofa distortion-compensated signal (a transmit signal after the distortioncompensation thereof) has exceeded a limit value, gain is enlarged so asto reduce the value of the difference between the transmit signal and afeedback signal from a transmission power amplifier, thereby controllingthe amplitude of the feedback signal to prevent an increase in adistortion compensation coefficient. As a result, distortioncompensation can be performed in stable fashion, without losingsignificant components (amplitude and phase) of the signal, through asimple arrangement. If gain G is fixed at a constant value greater than1, amplitude can be controlled in a simple manner. Further, if gain G isput into table form beforehand and control is exercised in conformitywith the transmit signal, control for preventing the occurrence ofamplitude and phase distortion can be performed more finely.

Further, in accordance with the present invention, gain G is controlledbased upon the level of the transmit signal only when the differencebetween the transmit signal before the distortion compensation thereofand the distortion-compensated signal has taken on a large value. As aconsequence, gain is controlled upon making sure that such control istruly necessary, and gain is not controlled when control is unnecessary.This makes possible finer control to suppress amplitude and phasedistortion.

Further, in accordance with the present invention, when the differencebetween the transmit signal before the distortion compensation thereofand the distortion-compensated signal has exceeded a threshold value,updating of the distortion compensation coefficient is halted. Thismakes it possible to prevent a distortion compensation coefficient fromtaking on a value of doubtful effectiveness.

Further, in accordance with the present invention, the size of aparameter μ for updating a distortion compensation coefficient iscontrolled based upon the difference between the transmit signal beforethe distortion compensation thereof and the distortion-compensatedsignal. This makes it possible to shorten convergence time while takinginto consideration the stability of the convergence of a linearizer. Forexample, if a situation arises in which the above-mentioned differenceexceeds a threshold value and divergence begins to develop in thedistortion-compensation control loop, the value of p is increased toprevent such divergence immediately. If the difference decreases, μ isreduced accordingly to effect a return to a steady value. As a result,control can be carried out in such a manner that the amplitude of thedistortion-compensated signal will not exceed a limit value whileconvergence and stability are maintained.

Further, the present invention is applicable to (1) a first distortioncompensation method of multiplying a transmit signal by a distortioncompensation coefficient and inputting the product to a distortiondevice, and (2) a second distortion compensation method of generating,as an error signal, the difference between a transmit signal and asignal obtained by multiplying a referential signal (the transmitsignal) by a distortion compensation coefficient, DA-converting theerror signal and a main signal (transmit signal) separately, combiningthe converted signals and inputting the result to a distortion device.

Further, in accordance with the present invention, the present inventionis applicable to a single-carrier transmitter and to a multicarriertransmitter.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

1. A distortion compensating apparatus for compensating for distortionof a transmission power amplifier, comprising: a memory for storingdistortion compensation coefficients, which are for compensating fordistortion of the transmission power amplifier, in association withpower p of a transmit signal; a predistortion unit for subjecting thetransmit signal to distortion compensation processing using a distortioncompensation coefficient that conforms to the power p of the transmitsignal; a DA converter for converting a digital transmit signal, whichhas been subjected to distortion compensation processing, to an analogsignal and inputting the analog signal to the transmission poweramplifier; a distortion compensation coefficient calculation unit forcalculating a distortion compensation coefficient based upon thetransmit signal before the distortion compensation thereof and afeedback signal fed back from an output side of the transmission poweramplifier; a comparison unit for judging whether the transmit signal,which is subjected to the distortion compensation processing using thedistortion compensation coefficient calculated by said distortioncompensation coefficient calculation unit, exceeds a dynamic range ofsaid DA converter; a distortion compensation coefficient correction unitfor correcting the distortion compensation coefficient in such a mannerthat the transmit signal that has been subjected to the distortioncompensation processing does not exceed the dynamic range of said DAconverter; and a distortion compensation coefficient updating unit forupdating a distortion compensation coefficient, which has been stored insaid memory, by the distortion compensation coefficient that has beencorrected.
 2. A distortion compensating apparatus for compensating fordistortion of a transmission power amplifier, comprising: a memory forstoring distortion compensation coefficients h(p), which are forcompensating for distortion of the transmission power amplifier, inassociation with power p of a transmit signal x(t); a predistortion unitfor subjecting the transmit signal x(t) to distortion compensationprocessing using a distortion compensation coefficient that conforms tothe power p of the transmit signal x(t); a DA converter for converting adigital transmit signal, which has been subjected to distortioncompensation processing, to an analog signal and inputting the analogsignal to the transmission power amplifier; a distortion compensationcoefficient calculation unit for calculating a distortion compensationcoefficient h_(n+1)(p) based upon the transmit signal x(t) before thedistortion compensation thereof and a feedback signal fed back from anoutput side of the transmission power amplifier where _(n+1) means anewly calculated value; and a table for storing, in advance inassociation with combinations of |x(t)|² and h_(n+1)(p), distortioncompensation coefficients h_(n+1)(p)′ obtained by correcting thedistortion compensation coefficient h_(n+1)(p) in such a manner thatpower Pa of a transmit signal, which has been obtained by subjecting thetransmit signal x(t) to distortion compensation processing using adistortion compensation coefficient h_(n+1)(p) will become smaller thanan upper-limit power Pmax, and for storing as is in advance, inassociation with combinations of |x(t)|² and h_(n+1)(p) distortioncompensation coefficients h_(n+1)(p) when the power Pa of the transmitsignal that has been subjected to distortion compensation is less thanthe upper-limit power Pmax; and a distortion compensation coefficientupdating unit which, when a distortion compensation coefficient has beencalculated by said distortion compensation coefficient calculation unit,is for obtaining, from said table, a distortion compensation coefficienth_(n+1)(p)′ that conforms to a combination of the calculated distortioncompensation coefficient h_(n+1)(p) and the power |x(t)|² of thetransmit signal x(t), and storing this distortion compensationcoefficient h_(n+1)(p)′ in association with the power in said memory. 3.A distortion compensating apparatus for compensating for distortion of atransmission power amplifier, comprising: a memory for storingdistortion compensation coefficients h_(n)(p) which are for compensatingfor distortion of the transmission power amplifier, in association withpower where n means a current value of a transmit signal; apredistortion unit for subjecting a transmit signal x(t) to distortioncompensation processing using a distortion compensation coefficient thatconforms to the power p of the transmit signal; a distortioncompensation coefficient calculation unit for calculating a distortioncompensation coefficient based upon the transmit signal x(t) before thedistortion compensation thereof and a feedback signal fed back from anoutput side of the transmission power amplifier; a distortioncompensation coefficient updating unit for updating a distortioncompensation coefficient, which has been stored in said memory, by thedistortion compensation coefficient that has been corrected; and a tablefor storing, in advance in association with combinations of |x(t)|² andh_(n)(p) distortion compensation coefficients h_(n)(p)′ obtained bycorrecting the distortion compensation coefficient h_(n)(p) in such amanner that power Pa of the transmit signal, which has been obtained bysubjecting the transmit signal x(t) to distortion compensationprocessing using a distortion compensation coefficient h_(n)(p) willbecome smaller than an upper-limit power Pmax, and for storing as is inadvance, in association with combinations of |x(t)|² and h_(n)(p),distortion compensation coefficients h_(n)(p) as h_(n)(p)′ when thepower Pa of the transmit signal that has been subjected to distortioncompensation is less than the upper-limit power Pmax; wherein adistortion compensation coefficient h_(n)(p) that conforms to power|x(t)|² of the transmit signal x(t) is read out of said memory, adistortion compensation coefficient h_(n)(p)′ that conforms to acombination of |x(t)|² and h_(n)(p) is read out of said table and thisdistortion compensation coefficient is input to said predistortion unit.4. A distortion compensating apparatus for compensating for distortionof a transmission power amplifier, comprising: a memory for storingdistortion compensation coefficients, which are for compensating fordistortion of the transmission power amplifier, in association withpower p of a transmit signal; an error signal generator for reading adistortion compensation coefficient that conforms to the power p of thetransmit signal out of said memory, subjecting the transmit signal todistortion compensation processing using this distortion compensationcoefficient and outputting an error signal, which is the differencebetween the transmit signal obtained by being subjected to distortioncompensation processing and the transmit signal before the distortioncompensation processing thereof; a DA converter for converting the errorsignal to an analog signal and outputting the analog error signal; acombiner for adding the output of said DA converter to an analogtransmit signal and inputting the resultant signal to the transmissionpower amplifier; a distortion compensation coefficient calculation unitfor calculating a distortion compensation coefficient based upon thetransmit signal before the distortion compensation thereof and an outputsignal of the transmission power amplifier; a comparison unit forjudging whether the transmit signal which is subjected to the distortioncompensation processing using the distortion compensation coefficientcalculated by said distortion compensation coefficient calculation unit,exceeds a dynamic range of said DA converter; a distortion compensationcoefficient correction unit for correcting a distortion compensationcoefficient in such a manner that the error signal does not exceed thedynamic range of said DA converter; and a distortion compensationcoefficient updating unit for updating a distortion compensationcoefficient by storing the corrected distortion compensation coefficientin said memory in association with the power p of the transmit signal.5. A distortion compensating apparatus for compensating for distortionof a transmission power amplifier, comprising: a memory for storingdistortion compensation coefficients h(p), which are for compensatingfor distortion of the transmission power amplifier, in association withpower p of a transmit signal; an error signal generator for reading adistortion compensation coefficient that conforms to the power p of thetransmit signal out of said memory, subjecting the transmit signal todistortion compensation processing using this distortion compensationcoefficient and outputting an error signal, which is the differencebetween a further transmit signal obtained by being subjected todistortion compensation processing and the transmit signal before thedistortion compensation processing thereof; a DA converter forconverting the error signal to an analog signal and outputting theanalog error signal; a combiner for adding the output of said DAconverter to an analog transmit signal and inputting the resultantsignal to the transmission power amplifier; a distortion compensationcoefficient calculation unit for calculating a distortion compensationcoefficient based upon the transmit signal before the distortioncompensation thereof and an output signal of the transmission poweramplifier; a table for storing, in association with a distortioncompensation coefficient h_(n+1)(p) that has been calculated by saiddistortion compensation coefficient unit where _(n+1) means a newlycalculated value, a distortion compensation coefficient h_(n+1)(p)′ thathas been corrected beforehand in such a manner that the square of thedistortion compensation coefficient h_(n+1)(p) will become smaller thanthe square |h(p)_(MAX)|² of a set maximum distortion compensationcoefficient; and a distortion compensation coefficient updating unitwhich, when a distortion compensation coefficient has been calculated bysaid distortion compensation coefficient calculation unit, is forobtaining, from said table, the corrected value h_(n+1)(p)′ of thedistortion compensation coefficient that conforms to the calculateddistortion compensation coefficient h_(n+1)(p), and storing thisdistortion compensation coefficient h_(n+1)(p)′ in association with thepower in said memory, thereby updating the distortion compensationcoefficient.
 6. A distortion compensating apparatus for compensating fordistortion of a transmission power amplifier, comprising: a memory forstoring distortion compensation coefficients h_(n)(p), which are forcompensating for distortion of the transmission power amplifier, inassociation with the power p of a transmit signal where n means acurrent value; an error signal generator for subjecting the transmitsignal to distortion compensation processing using a distortioncompensation coefficient that conforms to the power p of the transmitsignal and outputting an error signal, which is the difference betweenthe transmit signal obtained by being subjected to distortioncompensation processing and the transmit signal before the distortioncompensation processing thereof; a DA converter for converting the errorsignal to an analog signal and outputting the analog error signal; acombiner for adding the output of said DA converter to an analogtransmit signal and inputting the resultant signal to the transmissionpower amplifier; a distortion compensation coefficient calculation unitfor calculating a distortion compensation coefficient based upon thetransmit signal before the distortion compensation thereof and an outputsignal of the transmission power amplifier; a distortion compensationcoefficient updating unit for updating a distortion compensationcoefficient by storing the calculated distortion compensationcoefficient in said memory in association with the power p of thetransmit signal; and a table for storing, in association with adistortion compensation coefficient h_(n)(p) that has been calculated bysaid distortion compensation coefficient unit, a distortion compensationcoefficient h_(n+1)(p)′ that has been corrected in such a manner thatthe square of the distortion compensation coefficient h_(n)(p) willbecome smaller than the square |h(p)_(MAX)|² of a set maximum distortioncompensation coefficient; wherein a distortion compensation coefficienth_(n)(p) that conforms to power |x(t)|² of transmit signal x(t) is readout of said memory, the distortion compensation coefficient h_(n)(p)′that conforms to the distortion compensation coefficient h_(n)(p) isread out of said table and this distortion compensation coefficient isinput to said error signal generator.
 7. A distortion compensatingapparatus for compensating for distortion of a transmission poweramplifier, comprising: a memory for storing distortion compensationcoefficients, which are for compensating for distortion of thetransmission power amplifier, in association with power p of a transmitsignal; a predistortion unit for subjecting the transmit signal todistortion compensation processing using a distortion compensationcoefficient that conforms to the power p of the transmit signal, andinputting the resultant signal to the transmission power amplifier; adistortion compensation coefficient calculation unit for calculating adistortion compensation coefficient based upon a difference between thetransmit signal before the distortion compensation thereof and afeedback signal fed back from an output side of the transmission poweramplifier, and updating a distortion compensation coefficient, which hasbeen stored in said memory, by the distortion compensation coefficientthat has been calculated; and an amplitude controller for controllingamplitude of the feedback signal by changing a gain based upon amplitudeor the power p of the transmit signal before the distortion compensationthereof.
 8. A distortion compensating apparatus for compensating fordistortion of a transmission power amplifier, comprising: apredistortion unit for subjecting a transmit signal to distortioncompensation processing using a distortion compensation coefficient thatconforms to power p of the transmit signal; a DA converter forconverting a digital transmit signal, which has been subjected todistortion compensation processing, to an analog signal and inputtingthe analog signal to the transmission power amplifier; a distortioncompensation coefficient calculation unit for calculating a distortioncompensation coefficient based upon the transmit signal before thedistortion compensation thereof and a feedback signal fed back from anoutput side of the transmission power amplifier; a comparison unit forjudging whether the transmit signal which is subjected to the distortioncompensation processing using the distortion compensation coefficientcalculated by said distortion compensation coefficient calculation unit,exceeds a dynamic range of said DA converter; and a distortioncompensation coefficient correction unit for correcting the distortioncompensation coefficient in such a manner that the transmit signal thathas been subjected to the distortion compensation processing does notexceed the dynamic range of said DA converter.
 9. A method forcompensating for distortion of a transmission power amplifier,comprising steps of: subjecting a transmit signal to distortioncompensation processing using a distortion compensation coefficient thatconforms to power p of the transmit signal; converting a digitaltransmit signal, which has been subjected to distortion compensationprocessing, to an analog signal using a DA converter and inputting theanalog signal to the transmission power amplifier; calculating adistortion compensation coefficient based upon the transmit signalbefore the distortion compensation thereof and a feedback signal fedback from an output side of the transmission power amplifier; judgingwhether the transmit signal which is subjected to the distortioncompensation processing using the distortion compensation coefficientcalculated by a distortion compensation coefficient calculation unit,exceeds a dynamic range of said DA converter; and correcting thedistortion compensation coefficient, which has been calculated by saiddistortion compensation coefficient calculation unit, in such a mannerthat the transmit signal that has been subjected to the distortioncompensation processing will not exceed a dynamic range of said DAconverter.